Lefteris Kolleros & Alan P. Parkes

Computer-assisted dance teaching via multiple representations.

Lefteris Kolleros & Alan P. Parkes: Computer-assisted dance teaching via multiple representations", 16th International Congress on Dance Research, Corfu, Greece, 30/10-3/11, 2002.

Abstract

Computer technologies allow for a larger range of representations in presenting learning material. Multiple representations have been used successfully in a wide variety of educational applications with special emphasis on mathematics, physics and in incorporating simulations of engineering systems. Their strengths and weaknesses have been widely examined. Furthermore, aspects of education that were traditionally human-centered now involve computers to assist and improve the educational process. Computers may be capable of simulating human conditions in mechanistic ways, but there are obviously unanswered questions about whether they can contribute to the educational process in purely physical domains such as dance. This report describes the background and overall framework of a project that prototyped and evaluated an interface intended to provide a learning environment for a specific dance. The interface features multiple representations of the dancing steps. Keywords: multiple representations, co-ordination, dance teaching, problem solving, interactive carpet.

1. Background

1.1 Multiple Representations

Representations fall under two basic categories: internal and external. Internal (mental) representations correspond to the information stored in the brain whereas external representations are physical manifestations of this information in linguistic, graphical or other forms. For the remainder of the passage, the term representation will refer to external representations. Multiple representations are differing representations of the same entity. Obviously, the information carried by one representation may overlap with the information carried by other representations. The representations may either be co-present or presented alternatively. They may allow users to construct their own representations, or may present learners with the designer’s choice of representation.

The main advantages of learning with multiple representations can be summarised in the following lines:

- Multiple representations can be used to place different emphasis on aspects of complex domains and may be useful where one representation would be insufficient to carry all the intended information about the domain.

- With multiple representations, we are no longer limited by the strengths and weaknesses of one particular representation. Even within the same type of representation (e.g. graphical representations), one may be more efficient than another.

- Use of co-ordinated representations may increase the speed of learning and enhance the integration of information derived by different representations. However, co-ordination alone does not guarantee an improvement in knowledge transfer.

- When information is highly visual, a learner can encode the information into long-term memory using both a verbal and a visual trace. This redundant encoding increases the probability of retrieval because if one memory trace is lost (whether visual or verbal) the other sometimes remains available (Rieber 1990).

1.2. Dance

1.2.1. Greek dances

“The complex intertwining of power and pleasure in gender and sexual relation, the social shaping of the human body, the ambiguities of social experience, all examined in the context of an activity that I find fascinating. The Greek dance.” Jane K. Cowan (1992). Greeks experience and emit a wide range of feelings through dancing. The social context is an indispensable element of this process and it usually involves dancing in pairs, in a circle (by creating an open chain while dancers hold hands) or merely observing. Greek dances usually consist of repetitions of the same basic sequence of steps. There are differences in style between male-female dancing and the lead dancer is usually assigned with a special role.

1.2.2 Dance tutoring

The human centred methods of dance teaching, especially when applied to inexperienced dancers, separate the steps from the music in order to simplify the learning process. First, novices try to understand how a step is performed (without music). When they assimilate it, they proceed to next step. Whenever they are able to perform all steps, then music is applied. Moreover, step counting has been used extensively in dance teaching since it is very helpful for linking each step with the previous ones and mastering the rhythm. If used appropriately, computers may act complementary to the dance tutor and motivate learners to participate energetically in dancing. In the current project, we try to simulate elements of human-centred dance teaching methods in the computer-assisted learning of dance. At the same time, we incorporate techniques that are only feasible in a computer-based learning environment.

1.2.3. Dancing and technology

Dance-related use of computers falls under four main headings:

1) Creative choreographic experimentation – assisting choreographers and others to experiment with body movement and create choreographies (usually in a 3-D environment).

2) Implementation of dance notation systems which may intend to accomplish some or all of the following tasks: a) Teach dance notation. b) Create and edit dance notation scores. c) Interpret existing notation scores into “movies” of figures performing the dance sequences.

3) More radical attempts heading in various directions, using emerging technologies. A characteristic set-up includes one or more set of sensors, worn on the body or placed on the stage, that transmit information about the performer’s actions to a computer. Software is then used to interpret the data, providing the dancer direct control of several types of media including: Music synthesisers, Lighting, Digital audio effects, Video.

4) Applications intending to preserve and present the folk musical and dancing tradition. These are not usually in depth presentations and most times they are based on simple multimedia and database technology.

1.3 The potential of multiple representations in dance teaching

Dance is a many-sided expression of the human nature. It is impossible to capture a complete description of a dance just by using a single representation medium (e.g. video). Someone could say: “Dancers have always learned from watching others; they need only to watch the film”. However, there have found to be drawbacks to video learning, the greatest of which seems to be the viewer’s inability to see specifically what happens on the screen. According to Guest (1984), the viewer receives mainly an impression from the screen, rather than extracting specific or detailed information. Even if we recorded a dancer from the back, we would still get an incomplete total image of the dance as we would still lack views from other sides of the body (e.g. front). In the case of figure drawings, we would not only have this problem, but also the problem of showing the third dimension. Additionally, many movements are not pictorial in nature, and thus it is too difficult to represent them faithfully with figure drawing. As far as words are concerned, though they are not the ideal means to describe movement, they are a vital means of communication in learning to understand and master movement. The various representations should be combined in such a way that they complement each other in order to produce an optimum result. If books and filming have been helpful in communicating the dance knowledge, the PC’s capabilities of interaction and integration of the various media can bring significant developments to dance teaching.

2. Aims

The milestones of the project can be summarised in the following lines:

2.1. The prototype interface provides a learning environment for a specific dance. We chose to experiment with Greek dance Kalamatianos due to its pan-hellenic popularity and relative simplicity. The interface targets to novice dancers with average computing experience.

2.2. The prototype is based on multiple co-ordinated representations of the dancing steps.

2.3. The prototype underwent evaluation using real users.

3. Implementation

3.1. Multiple representations

The representations of the dancing steps used in the interface are: Video; Drawings; Thumbnails (of the drawings and the video frames); Footprints; Word description. The pictorial representations (Drawings, Thumbnails, Video) are available in various views (Back, Front, Side) in order for the user to form a complete image of the dance. The top view is actually provided by the footprints representation. User has the possibility for the pictorial representations to view all instances of the representation (s)he wishes, up to a specific step in order to secure prior knowledge and integrate the corresponding step with the previous ones.

3.1.1. Video

Video was introduced since it represents the dance faithfully. Video has the advantage of combining two different media: audio and images combined and presented in a flowing order. Hence, the users will not simply be able to assimilate the steps by watching the pictorial aspect but they will also be able to correlate the steps with the music. The video reresentation shows the still frame that corresponds to the current step. The user is able to switch to various views (back, front, side) and play the video clip of the currently active view up to the current step. Video clips are also provided:

- for showing the dance along with the counting of the steps in order for the users to associate the steps with the rhythm, and

- for displaying experienced dancers performing the dance in order to enable the user to observe elements of the dance such as the style, how the dancers hold hands, the social context that surrounds the dance, the role of the leading dancer and many other parameters that all together constitute the natural environment within which the dance takes place.

3.1.2. Drawings

Further pictorial representation is drawings of the various poses of the dancer’s body. Apparently, video and drawings partially overlap, with the video providing more information. Drawings can with difficulty approximate the natural pose of the body as this is displayed in the video frames. Nevertheless, the designer can be more flexible and apply various techniques to drawings that make them both more enjoyable and effective. The drawings representation provides one mode for showing the whole body and another for displaying only the legs (enlarged in order to amplify the impact), since legs carry vital information about the way in which the dance is performed.

3.1.3. Thumbnails

Thumbnails in general are small versions of original images and thus less demanding in terms of screen space and computer resources. The thumbnails representation display all steps up to current step in the form of drawings or video frames.

3.1.4. Footprints

Another pictorial representation that has been used extensively in books in order to describe dancing, are the footprints of the dancing steps. The footprints representation provides an option for displaying only the footprints that correspond to the current step and another option for displaying all footprints up to the current step. In the latter case, the user is able to add connecting arrows between the successive right (left) footprints indicating the orbit of the right (left) foot.

3.1.5. Word description

Apart from the pictorial representations, word description can be used in order to precisely describe the movement and complement the pictorial representations of the dance. The word description as implemented in the current interface includes three options: One option for displaying the written description, one for playing back the verbal description of the current step and one for enabling the verbal counting of the steps.

3.2. Additional functionality

3.2.1. Music

The music window includes an option for playing back the default song accompanied by a voice counting the steps in time as the music plays, in order to help the user to assimilate the timing of the steps. A list of songs is available, since dancers should listen to various songs in order to master the rhythm.

3.2.2 Animation

Animation in the current case exceeds common animation since it involves the simultaneous presentation of the various representations (even of the non-pictorial ones). It is based on showing and hiding open representations (thus creating the illusion of movement) up to the current step. The user is able to apply animation with or without music. In the first case, all open representations are animated to the rhythm in order for the user to associate the steps with the music. In the second case, the timing gap between successive steps is the same for all steps but the user is able to increase or decrease the speed with which the representations switch from one step to the next according to preference. This speed is expected to increase as the skill and knowledge of the user increases. An option is provided for automatically restarting the animation when the final step is reached.

3.2.3. General information

The users also have access to related information in order to form a more complete image about the dance. According to Kyriakou (1993), learning about the general context of a folk dance (how the dance was created, in which occasions it is danced and from what people etc.) is vital in order to understand and enjoy the dance more deeply. A Web browser therefore enables users to access general information about the dance available through local and WWW pages.

3.3. Problem solving

The interface should not only enable the users to observe, but also to practise (to some extent), the presented material. Quizzes are a suitable way of adding interactivity to the application. The quizzes cover the actual steps, the timing between them and even other things that are vital for learning the dance, such as the way that the dancers hold hands to form a circle. Appropriate error messages and feedback in general are provided in order to help the users to correct their deficiencies and improve their dancing skills.

3.3.1. Hands quiz

In this quiz, a number of images are presented in random order to the user and (s)he has to select the right one in order to pinpoint what is the proper way for the dancers to hold hands.

3.3.2. Timing quiz

The timing quiz was introduced in order to help the users to grasp the timing of the steps. There is a small introduction to the song and then a voice counts the first set of steps in time in order to introduce the user smoothly to the rhythm. The user is responsible for pointing where the steps fall in time as music plays by clicking the appropriate button. Error messages are provided for each of the following cases: The user was too slow; The user was too fast; The user did not “count” at all; The user stopped “counting” after a specific step.

3.3.3. Drag & drop quiz

In the drag & drop quiz, images of the steps are presented in wrong (random) order and the user has to re-arrange them appropriately using drag and drop in order to pinpoint the right order. The user can switch between various representations (Drawings, Frames) and views (Back, Front, side).

3.3.4. Magic carpet quiz

3.3.4.1. Set-up and functionality

The fourth quiz is based on the footprints’ representation. The user is presented (on screen) with a collection of footprints. (S)He is supposed to select the correct footprints for each step using the mouse. In order to make the quiz more interactive in a way that the user actually performs the steps upright and not simply by using a mouse, switches were stuck beneath a piece of carpet in positions that correspond to the steps of the dance. Footprints were stuck over the carpet exactly over the switches (see figure 1). The left and right footprints were painted in different colours and generally the carpet was an exact copy of the window on the screen. The switches were connected to the keys of the keyboard. Hence, stepping on a switch was equivalent to a keypress (see figure 2). The quiz can be invoked with or without music. When the quiz is invoked without music, the user has to step on the correct footprint for each step. If the quiz is invoked with music then, the user has to actually dance on the footprints. This time, (s)he does not only have to get the steps correct but also the timing. Error messages have been included for the same cases as in the timing quiz.

3.3.4.2. Limitations and suggested improvements

The carpet quiz presents the following limitations:

1) It cannot detect steps whenever the feet do not touch the floor. Similarly, it cannot detect movement of the hands and generally of the upper part of the body. A possible solution is to combine the carpet with real time image processing using video cameras or other motion capture hardware. The system will be able to decide whether the dancer performs the steps correctly using pattern recognition techniques and will feedback errors to the user in real time.

2) The carpet can be used only for a limited number of steps. This problem could be alleviated if a circular carpet is used (since the current dance is circular)

3) It is not possible to achieve an ideal distance separating subsequent footprints as far as user physical dimensions are concerned.

4) The users focus mainly on treading exactly on the footprint rather than on dancing naturally.

Magic carpet is consistent with the way in which the other quizzes were implemented. However, points three and four suggest that a totally interactive floor should also be implemented, capable of capturing the learner’s footpress at any point. The problem in this case is that the switches are expected to exceed the number of the keys available on the keyboard. The input of the switches should be therefore directed to the computer not through the keyboard but through another input device capable of reading the co-ordinates of footpresses to the controlling program.

3.4. The environment

An appropriate environment was set up for displaying and controlling the various representations. The representations are placed in appropriate positions in the screen. The most important ones (i.e. the most effective) were placed in the most noticeable places. In addition to this, representations that are related to (or complement) each other (such as the drawings and the thumbnails) are placed in close proximity to enable the user to compare the representations and draw conclusions more easily. Two toolbars are provided on top of the screen: one for opening (closing) the representations and another for switching to different steps of the dance. Whenever the user switches to another step, all the representations are modified to match the current step (see figure 3).

4. Evaluation

4.1. Objectives

The evaluation examined the overall effectiveness of the interface, and of each one of the representations used, the usability of the interface and the detection of errors in the functionality of the program. It is fairly obvious that there are no clear dividing lines between these objectives as they may affect each other in various ways.

4.4.1. Overall effectiveness of the interface

The actual aim of the interface was to provide a learning environment for a specific dance. The evaluation therefore tested whether, and if so, to what extent the subjects acquired the corresponding skills.

4.1.2. Multiple representations

The present evaluation measures the effectiveness of the current implementation of each representation. These measures may provide a rough indication about the general effectiveness of each representation but these indications cannot be trusted unless further research establishes them. Further factor affecting the effectiveness of a representation is how well the representation works in combination with the representations and, more generally, with the functionality available in the interface. As a consequence, a representation cannot be easily separated from the surrounding interface. The co-ordinated use of representations makes the separation of each representation from the others even more difficult. The effectiveness of a representation also depends on subjective criteria such as each user’s personal tastes, learning preferences, capabilities and related experience. For example, a visualiser may find drawings and video more effective, whereas a verbaliser may prefer the word description of the steps.

An alternative approach could be to restrict the interaction to a single representation. This approach would offer more precise results about the effectiveness of a particular representation but the user would have to interact within a single representational environment. Hence, there would be no clues about the effect of multiple representations in the learning process. This approach should therefore be used only when the objective is to test the effectiveness of a single representation in isolation from others, and not to test the effectiveness of a representation within a multi-representational environment.

4.2. Methods

The methods used (in the timing order that they were applied) were the following:

- Pre-questionnaire: Aiming to obtain the profile of the user in terms of computing and dancing experience.

- Observation (in the forms of video recording and note-taking):

- Video recording was used to capture details of the interaction and provide a permanent record that could be studied many times later. The aspect of the interaction captured was the screen in order to avoid affecting the performance/concentration of the user through the intrusive use of camcorders.

- Note taking was used to record the most important moments of the interaction as well as aspects of the interaction that the camera could not capture such as the facial expression of the user, the use of the keyboard and to provide an alternative solution in case of problems with the video recording.

- Post-questionnaire: The post-questionnaire was intended to gain an initial feedback from the user.

- Interview: The interview intended to expand on the answers provided in the questionnaire and provide answers to more general questions that could not be included in the questionnaire.

4.3. Tasks

The set of tasks consisted of three parts. The first part was actually the interaction with the interface. During this task the user was expected to acquire the knowledge and skills required in the next parts. The second part actually involved the execution of the quizzes. The tasks were designed to be of progressive difficulty. In the third part, the users were asked to perform the steps without music and if they succeeded to repeat with music. Finally, they were asked to hold the observer’s hand in the appropriate way.

4.4. Test group

The subjects were novice dancers with average computing experience aged between 18-30 years. The reason for the last condition is that learning speed of dancing steps varies with age (Raftis, 1993). Children, for example, have proven to be the fastest learners of dancing steps throughout all ages. The test group consisted of six subjects. An extremely small number of subjects took part in the evaluation. The results should therefore be generalised with caution.

5. Conclusions

5.1. Multiple representations

The most effective representations proved to be the footprints when all steps up to current step are presented. The video frames (static) and video clips were found to be the next most effective representations. The animation was also considered effective because the representations could actually be observed in action and they were therefore presented in a more natural form. Less effective representations were found to be the drawings and the word description of the steps. The drawings were not found particularly effective mainly because they did not represent the movement as clearly and naturally as the video frames. The word description could not compete with pictorial representations mainly because “One image worth one thousand words”. When the word description was used, the users actually had to translate the words into images of steps whereas with pictorial representations the visual information did not have to be pre-processed in such a way. Pictures are also easier to remember, since memory for pictures has been shown to be superior than memory for text. On the other hand, on certain occasions, subjects used the word description in order to appreciate exactly what they were supposed to do. For example, when the pictorial representations did not describe the movement clearly, subjects used the word description to reassure themselves and understand precisely how the step is performed.

The verbal description proved to be more helpful than the written description of the steps. As one subject explained, it was easier to watch a pictorial representation and listen at the same time to the description of the step. Two different senses were used in a complementary way thus making better use of cognitive resources. Counting was generally found to be helpful in remembering the steps. No representation was found to be ineffective, which may possibly suggest that all representations had something to offer. In fact, even the less effective representations were considered very effective by certain subjects.

The most effective combination of representations were considered to be the footprints and the video clips, mainly because the footprints taught the steps and the video clips illustrated the dance well (the whole body naturally following the rhythm). The most effective view was the back view, and the least effective the front view. As most subjects stated, it was easier to grasp the steps when they watched the figure from the back, because the figure moved in the same direction as did they. Generally the effectiveness of each representation was directly proportional to the derived enjoyment. As most subjects stated, learning is generally more effective when it is enjoyable. This may be applied vice versa. A representation is found more enjoyable when it is more effective and therefore helps users to increase their knowledge and skill.

Most of the subjects did not find the co-ordinated use of multiple representations confusing as they could easily focus on the representation(s) in which they were interested. All subjects stated that they would prefer a multi-representational environment to a single representational one, since one representation (even the most effective) cannot alone carry all the necessary information. Hence, the time and effort spent in developing multi-representational environments are worthwhile. This finding apparently applies only to dance teaching and ought not to be generalised to other domains.

5.2. Quizzes

The timing quiz was considered of neutral difficulty. The drag & drop quiz along with the carpet quiz (when performed on the carpet with music), considered to be the most difficult to accomplish. No quiz was found to be very easy or too difficult. The carpet quiz was found to be the most stimulating. It was also proved quite effective for learning the steps.

5.3. Overall

All the subjects learnt at least half of the steps. Two of them performed all the steps without music correctly. One subject managed to perform the steps with music. The average rating of the interface is presented in figure 4. The interface was demonstrated to be effective especially if we consider the limited time of interaction (approximately one hour) and the low dancing experience of the subjects. As a result, multiple representations in dance learning environments can be se-considered to be effective.

An advantage of computer-assisted dance training is that the computer provides a permanent record that the user will be able to use again and again, at his/her own pace, at any time. It is encouraging that subjects did n’t feel the need to be assisted by a human dance tutor during the interaction. However, it is difficult for the machine to achieve the adaptivity of the human dance tutor to the reactions of the learner. In addition to that, all subjects stated that the present interface can only complement human-centered methods of dance teaching. The computer may adequately present and teach a sequence of steps but the dancer can fully learn the dance only if (s)he dances it together with experienced dancers in order to develop perception, confidence and maturity. Ideally, the dance should be learnt in its natural environment (i.e. where the dance originated).

Learning a dance via the computer, even if this is an interactive process, lacks the characteristic of public exhibition. The social underdevelopment of the users is a general danger of learning from computers. The social context of folk dances is not only vital for the social development of the dancer but for the continuity of tradition as well. Tradition is not kept alive by watching on a screen what our ancestors used to do some years ago. Tradition is kept alive, when all the elements that constitute it, (the people, the dance, the customs), come actively to a reunion.

6. Additional possibilities

- The interface may include functionality for teaching the differences in style between male/female dancing and the variations executed by the first dancer.

- The techniques used were applied to a specific dance. More dances can be incorporated into the program in order to determine more reliable ratings for the interface and the representations.

- Systems should be developed in such a way that they cover all the possible levels of expertise (Fisher and Scharf 1998). Hence, when a user improves his knowledge and skills, (s)he won’t be forced to follow the same slow pace as when (s)he was novice. The present user interface was pre-determined to have as target novice dancers. However, it can be extended to adapt to different levels of expertise. Similarly it can be extended to cover users of various ages and computing experience.

7. References

Ainsworth, S., Wood, D. & Bibby, P. (1996): “Co-ordinating Multiple Representations in Computer Based Learning Environments”. In P. Brna, A. Paiva & J. Self (eds) European Conference on Artificial Intelligence in Education. 30 Sept. - 2 Oct. 1996, Lisbon, Portugal, pp 336-342.

Barker P.: “Multimedia computer assisted learning”, Kogan Page, London/Nichols Publishing, 1989, pp 118-138.

Cowan J.: “Dance and the Body Politic in Northern Greece”, PrincetonUniversity Press, 1992, pp 1, 104, 151, 152, 163, 198.

Cox R., Brna P.: “Supporting the Use of External Representations in Problem Solving: The Need for Flexible Leaning Environments”, Journal of Artificial Intelligence in Education, 1995, Vol. 6, No. 2/3, pp 239-302

De Jong T., Sarti L.: “Design and Production of multimedia and Simulation-based Learning material”, Kluwer Academic Publishers, 1994, pp 2-30.

De Jong T., Ainsworth S., Dobson M., van der Hulst A., Levonen J., Reimann P., Sime J.A., van Someren M., Spada H., & Swaak J. (In Press), “Acquiring knowledge in science and maths the use of multiple representations in technology based learning environments.”, Chapter 1 in Maarten van Someren, Peter Reimann, Els Boshuizen, Ton de Jong (eds) “Learning with Multiple Representations”. Amsterdam: Elsevier.

Edwards D. A., Holland S.: “Multimedia interface design in education”, NATO ASI Series, 1989, pp 177-194.

Fischer G., Scharff E.: “Learning Technologies in Support of Self-Directed Learning”, Journal of Interactive Media in Education, 1998, http://www-jime.open.ac.uk/

Guest A. H.: “Dance Notation: The process of recording movement on paper”, Dance Books, 1984, pp 8-14, 163-173

Guilcher J., M., Guilcher N., Kyriakou C., Zikos J., Panagioditou A., Panagiotopoulou A., Raftis A.: “Teaching Dance”, International Organisation of Folk Art, 7th International Conference on Dance Research, Portaria, 7-11 July 1993, Vol. 1, pp 14, 18, 22, 27-31, 52, 71, 73, 80, 83, 102-103, 106.

Mahia A. & Raftis A.: “Art and Tradition”, Dora Stratou Dance Theatre, November-December 1995, Vol. 24, pp 8-12.

Mayer R., Sims V.: “For Whom Is a Picture Worth a Thousand Words? Extensions of a Dual-Coding Theory of Multimedia Learning.” Journal of Educational psychology, 1994, Vol. 86, No. 3, pp 389-401.

Preece J., Rogers Y., Sharp H., Benyon D., Holland S., Carey T.: “Human Computer Interaction”, Addison-Wesley, 1997, pp 86-87, 698.

Raftis A.: “The World of Greek Dance”, FineDawn Publishers, 1987, pp 21, 24, 66, 82, 86, 88, 90, 92

Reinhard P., Hesse F.W., Hron A., Picard E.: “Manipulable graphics for computer-supported problem solving”, Journal of computer assisted learning, 1997, Vol. 13, pp 148-162

Repenning A., Ioannidou A., Ambach J.: “Learn to Communicate and Communicate to Learn”, Journal of Interactive Media in Education, 1998, http://www-jime.open.ac.uk/

Rieber L.: “Using Computer Animated Graphics in Science Instruction With Children”, Journal of Educational Psychology, 1990, Vol. 82, No. 1, pp 135-140.

Roubis G.: “Greek Dances”, Sbilias Publications, 1990, pp 152-159

Shawn T.: “Dance we must”, Haskell House Publishers, 1974, pp 60-61

Stratou D.: “The Greek Dances. Our Living with Antiquity”, Dora Stratou Dance Theatre, 1992, pp 13, 14, 17, 18, 29

Thomas J., Kellog W.:, “Minimising ecological gaps in interface design”, IEEE Software, January 1989, pp 78-86

8. List of figures

Figure 1: The interactive carpet

Figure 2: The interactive carpet from beneath

Figure 3: An instance of the interface at step “Two”

Figure 4: Average rating of the interface

The interface was implemented in the Enterprise Edition of Visual Basic 5.0

Dr. Alan P. Parkes

 

Lefteris Kolleros