as described above , the preferred embodiment of the present invention is designed to facilitate user understanding of a domain of knowledge underlying a knowledged - based system , such as a nuclear cardiology expert system . the following detailed description illustrates the manner in which the present invention can be used with a nuclear cardiology expert system , nuces , designed to aid physicians in offering a diagnosis on the basis of first pass cardiac functional images . however , it will also become clear that the present invention has uses well beyond a nuces type system . for example , it will be clear that the principles of the present invention can be used to construct explanation facilities for other expert systems , or to construct teaching or reference tools for exploring domains of knowledge . fig1 schematically illustrates how a system constructed according to the present invention can be used in connection with a nuclear cardiology expert system 10 ( nuces ). in such an expert systems first pass functional imaging of a patient &# 39 ; s heart , along with certain numeric patient data , provide a patient image base 12 and a patient data base 14 , which are analyzed via a knowledge - based system driver 16 which can draw on several knowledge bases 18 ( e . g ., image database , case library , diseases knowledge base , objects and classes knowledge base , etc .) to perform a diagnosis for the patient . the present invention embodies a computer - based method of organizing and representing domain knowledge that provides a browseable facility which can function as an explanation tool for expert systems , a reference tool or a teaching tool . referring to fig2 the explanation facility 20 includes a data processing system including an input device , a data structure defining a domain model 22 , and a user interface 24 . the domain model 22 is constructed to reflect the knowledge ( i . e ., the human cognitive processes ) of an expert in nuclear cardiology . the domain model 22 includes ( i ) at least one concept map relating to the expert &# 39 ; s view of first pass cardiac functional imagery , and ( ii ) supporting devices including mediating representations ( e . g ., audio devices , visual devices , video devices , other concept maps , repertory grids , textual devices ) connected with selected concepts forming nodes of the concept map . each of the mediating representations has an icon associated therewith which symbolizes the availability of that particular mediating representation at each node in a concept map . the user interface 24 is adapted to ( i ) display the domain model , or selected portions thereof ( including the concept maps and other mediating representations and the icons associated therewith ), and ( ii ) respond to inputs from the user to enable the user to enter the domain model and to interact with the supporting devices to understand and explore the domain model . thus , as the expert system is being used for performing a diagnosis , the user interface 24 enables the user to enter and explore the domain model 22 to understand the expert ( s )&# 39 ; reasoning in forming the diagnosis . on the other hand , the user interface 24 also enables the user to enter and explore the domain model 22 apart from the expert system , and to use the domain model 22 as a teaching or reference tool in connection with nuclear cardiology . a system according to the present invention can be implemented on a macintosh ™ quadra computer . the macintosh ™ quadra computer has extensive visual and sound capabilities that enable a user interface and domain model to be constructed as described herein , in accordance with the principles of the present invention . programming of the macintosh ™ quadra computer can be done in various programming languages , to implement the principles of the present invention . for example , the programming for creating the domain model and user interface described below were written in the &# 34 ; c &# 34 ; programming language . the components of the domain model and the user interface , which form the essence of the present invention are illustrated in fig5 - 14 . the domain model is based upon an expert view of a topic ( e . g ., nuclear cardiology ) and includes at least one concept map ( see the top level concept map in window 40 in fig5 and 6 ), and preferably several concept maps ( an ischemia concept map is shown in window 42 in fig7 ). each concept map is formed by a series of nodes ( see ischemia node 44 in the ischemia concept map shown in window 42 in fig7 ), and links between selected nodes ( see e . g ., links 46 , 48 in the concept map shown in window 42 of fig7 ). the nodes represent different concepts of the expert &# 39 ; s view of the topic . the links represent relationships between selected concepts , also according to the expert &# 39 ; s view of the topic . together , the nodes and links organize the concepts into a hierarchical framework representing at least a portion of the expert &# 39 ; s view of the topic . thus , in fig5 the top level concept map shown in window 40 represents an overview of the expert &# 39 ; s view of the domain wherein the node labeled &# 34 ; first pass radionuclide ventriculogram &# 34 ; represents the nuclear cardiology expert &# 39 ; s broadest concept relating to that topic . in fig7 the ischemia concept map shown in window 42 presents an overview of the expert &# 39 ; s view of ischemia . supporting devices are associated with selected nodes of each concept map . the supporting devices are preferably one or more mediating representations of a type taken from a group comprising ( i ) audio devices , ( ii ) visual devices , ( iii ) video devices ( e . g ., motion pictures ), ( iv ) textual devices , ( v ) repertory grids , and ( vi ) other concept maps . the mediating representations provide tools which expand the expert &# 39 ; s explanation of concepts forming part of the domain model and / or which direct the user to other related concepts ( or to other related mediating representations ), to enable the user to interact with the domain model , to understand and explore the domain model . an icon is associated with each mediating representation . fig1 illustrates the icons 50 , 54 , 58 , 62 , 64 , 68 which are associated with the preferred mediating representations for the nuclear cardiology domain model . fig1 also illustrates an icon 70 which represents another supporting device which is not a mediating representation , but instead enables backtracking through the domain model . the icon 50 symbolizes textual devices ranging from brief text - based descriptions of the relevant concepts ( as shown in window 52 of fig9 ) to indepth journal articles or segments from books . when a plethora of textual documents are available for a particular concept , they are organized in a concept map as shown in window 51 of fig8 . the icon 54 symbolizes images of various kinds , including drawings and photographs ( among others ) as shown in window 57 of fig1 . as with the other mediating representations , when a plethora of relevant visual images are available for a particular concept , they are organized in a concept map as illustrated in window 56 of fig1 a and 10b . in the example of fig5 - 14 , the images may comprise nuclear cardiology patient images which are digitized , grouped in accordance with the expert &# 39 ; s domain of knowledge , and disposed within respective windows in the domain model . the icon 58 symbolizes a video device ( e . g ., as shown in the window 60 in fig1 ). the video device appears as a window 60 with a view of the expert describing aspects of the domain model . the audio / visual device preferably comprises a digitized segment of a video recording . the video device can be produced using quicktime ( an extension of the macintosh ™ operating system ), and various audio / visual tools available in a macintosh ™ environment . in the example of fig5 - 14 , the video devices were produced using quicktime , a program called &# 34 ; media grabber &# 34 ;, marketed by raster ops corporation , santa clara , calif ., a raster ops 24xltv graphics board and a raster ops movie pak daughter board . the icon 62 symbolizes audio devices ( e . g ., sound recordings of an expert &# 39 ; s explanation of a part of the domain model ). the sound recordings can be readily integrated into the domain model , using tools readily available in a macintosh ™ environment . the icon 64 symbolizes a repertory grid ( e . g ., as shown in window 66 in fig1 ). a repertory grid is a knowledge acquisition / explanation tool comprising a grid constructed of elements identified by the expert as related to different concepts of the domain model and values which are incorporated into the grid to weight the expert &# 39 ; s view of those elements toward one or the other of the concepts , to help explain and / or illustrate certain aspects of an expert &# 39 ; s view of those elements and concepts . the icon 68 symbolizes other concept maps ( e . g ., as shown in windows 40 , 42 and 59 in fig5 , and 12 ). when the user selects this icon , a popup menu of other concept maps that are related to the particular concept under study is displayed . the user can now select one of these maps and be conveyed to that location in the domain model . for example , in fig6 if a user selects the &# 34 ; concept map &# 34 ; icon associated with the concept &# 34 ; ejection fraction &# 34 ; in the top level concept map shown in window 40 , a popup menu 69 is produced showing other concept maps which are related to the concept ejection fraction . the user can select one of those other concept maps to navigate to that concept map . concept maps are typically created by a knowledge engineer interviewing an expert on a topic , forming the concepts expressed by the expert into a hierarchical framework of nodes , and using links to connect the concepts according to the expert &# 39 ; s view of the topic . alternatively , the expert may construct the map by him or herself . finally , the icon 70 is a backtrack . by selecting this icon , users can return to their previous location in the domain model . the icon 70 is not a mediating representation , but functions as a supporting device which can be used in exploring the domain model . as can be seen by reference to fig5 - 14 , a node in a given concept map may have none or several mediating representations associated with that node . further , it should be noted that a window associated with a given mediating representation may have several supporting devices , including icons indicating the availability of other mediating representations associated with that particular concept under study . for example , in the &# 34 ; example images &# 34 ; window 57 illustrated in fig1 , the window includes icons indicating the availability of text , concept map , and video mediating representations . these icons and their associated mediating representations provide tools for enabling a user to acquire additional knowledge about the images in the window , or to move from that window to another place in the domain model to continue the user &# 39 ; s understanding and exploration of the domain model . still further , a concept map hierarchy 72 ( fig5 - 14 ) is provided which , in the preferred embodiment , is always available for viewing by a user . the concept map hierarchy 72 identifies the concept map in which the user is currently located within the hierarchy of concept maps in the domain model . the user interface displays the domain model , or portions thereof , and enables the user to enter the domain model and use the mediating representations to understand and explore different aspects of the domain model . for example , a display monitor associated with the macintosh ™ computer would display the different parts of the domain model ( e . g ., those parts of the domain model illustrated in fig5 - 14 ). the user interface enables the user to select a particular supporting device , e . g ., one of the icons associated with a mediating representation , to utilize that mediating representation to understand or explore the domain model . moreover , the user interface provides a tool which conveys the user into the domain model to begin the user &# 39 ; s interaction with the domain model . for example , during a consultation with the expert system ( as shown in window 87 in fig3 ) the user is offered an opportunity to request an explanation by selecting the &# 34 ; explain &# 34 ; button 93 . at this point , based on the context of the user &# 39 ; s interaction with the expert system , the user would be conveyed to a relevant location within the domain model to begin interaction with and exploration of the knowledge represented therein . furthermore , domain models of the sort made possible by the present invention can be used as a reference or teaching tool independent of their use with expert systems . the system includes an input device including a pointer device ( e . g ., a mouse , trackball , light pen , keypad , keyboard , etc .) offering the user a tool to move a cursor about the domain model , and to select lines , words , or icons to cause the system to display the mediating representations symbolized by the icon , or to convey the user to another part of the domain model , etc . the macintosh ™ environment , including its graphic user interface , provides convenient tools for producing and displaying the domain model and for enabling a user to enter and interact with the domain model , in order to understand and explore the domain model . the types of input devices described above are well known in the art , and should require no further explanation . fig5 - 14 illustrate part of an exploration of a domain model , to demonstrate the capabilities of the present invention . as shown in fig5 the user has been conveyed into the domain model at the broadest concept in the top level concept map illustrated in the window 40 . as illustrated , that broadest or most general concept is described by the expert as &# 34 ; first pass radionuclide ventriculogram &# 34 ;. referring to the concept map hierarchy window 72 , the user can see that he is located in the top level concept map . by viewing the top level concept map , the user can see the hierarchical framework used by the expert to describe broad concepts related to nuclear cardiology . the user also gets a view of the icon symbolizing the mediating representations associated with selected concepts forming nodes in the top level concept map . as illustrated in fig5 certain nodes ( e . g ., &# 34 ; general abnormalities &# 34 ;) have a large number of mediating representations associated with them . other nodes ( e . g ., ejection fraction ) may have fewer mediating representations associated with them . as illustrated in fig6 a user has selected the &# 34 ; concept map &# 34 ; icon associated with the &# 34 ; ejection fraction &# 34 ; node in the top level concept map . the popup menu 69 identifies to the user the names of the other concept maps which are related to the &# 34 ; ejection fraction &# 34 ; concept . as further illustrated , the user has selected the &# 34 ; ischemia &# 34 ; concept map in the popup menu 69 . the system then conveys the user to the ischemia concept map illustrated in window 42 in fig7 . while in the ischemia concept map , the user can view the expert &# 39 ; s hierarchical framework of concepts related to ischemia . further , as illustrated in fig7 the concept map hierarchy 72 gives the user a visual image of the fact that he is now in the &# 34 ; ischemia concept map &# 34 ;, and locates the &# 34 ; ischemia concept map &# 34 ; within the concept map hierarchy of the domain model . as illustrated in fig8 the user has selected the &# 34 ; text &# 34 ; icon under the broad concept , &# 34 ; ischemia &# 34 ;, in the ischemia concept map . as illustrated , rather than moving specifically to a body of textual material , the text icon first conveys the user to the ischemia text map illustrated in window 51 . the ischemia text map shows ( i ) a hierarchical framework of textual concepts related to ischemia , and ( ii ) textual mediating representations ( e . g ., text - based documents ) associated with certain of those concepts . by selecting the mediating representation associated with the textual concept labeled &# 34 ; textual description of ischemia &# 34 ; in the textual concept map of fig8 the system produces the window 52 with a body of text describing ischemia ( see fig9 ). further , as indicated by the available icons , the window 52 in fig9 offers the user several other mediating representations which would enable the user to further explore the domain model . fig1 a and 10b illustrates what happens if the user selects the &# 34 ; images &# 34 ; icon at the &# 34 ; ischemia &# 34 ; node of the ischemia concept map shown at window 42 in fig7 a and 10b . the window 56 is produced ( fig1 a and 10b ) showing a concept map identified as &# 34 ; ischemia images &# 34 ;, with mediating representations available at certain nodes in that concept map . when the user selects the node associated with the concept &# 34 ; limited hilat and / or inferior &# 34 ;, the window 57 is opened ( fig1 ) and example images related to the concept of limited high lateral or inferior blue fingers are shown , in that window , the user is offered several additional mediating representations to enable the user to further understand and explore the domain model , for example , if the user selects the &# 34 ; concept map &# 34 ; icon in the example images window 57 of fig1 , the user is conveyed to yet an additional window 68 ( fig1 ) showing a concept map entitled &# 34 ; nonspecific wma &# 34 ;. that concept map provides a hierarchical framework of additional concepts related to the images shown in the images window 57 ( fig1 ) which the user was exploring . thus , the user , after navigating through several concept maps , and through several mediating representations , and by using the mediating representations associated with that window , is conveyed to yet another concept map in the domain model , so that the user can continue his exploration of additional aspects of the expert &# 39 ; s domain of knowledge underlying the domain model . turning to fig7 and 13 , the user has selected the repertory grid icon associated with the ischemia node shown in window 42 in the ischemia map . the window 66 is opened providing the user with a repertory grid associated with the ischemia node . also , as seen by the icons displayed in that window , additional mediating representations are offered , as well as an opportunity to select an element or construct in the repertory grid to show the information choices for that item . finally , fig7 illustrate the mediating representation presented to the user when the user selects the video icon associated with the ischemia node shown in window 42 in the ischemia concept map . the window 60 is opened with a video segment in which the expert offers an explanation of the ischemia concept . moreover , although not illustrated in fig1 , the window 60 with the video segment could also include appropriate icons to allow further navigation of the domain model from that window . thus , from the foregoing discussion , it should be clear how a user can interact with the domain model , using the mediating representations to amplify concepts found in the domain model and / or to navigate to other related concepts . these tools allow the user to browse the domain model as a reference or teaching tool . moreover , they provide a picture of the knowledge underlying the domain model in such a way that the user can use the domain model to construct his / her own explanations . as explained above , when the present invention is associated with an expert system , the user interface can convey the user to an appropriate point in the domain model to enable the user to begin his / her interaction with the domain model . for example , at any time during a consultation with the expert system ( e . g ., as shown in fig3 ), or once the expert system has reached a diagnostic conclusion ( e . g ., as shown in fig4 ), the user is offered an opportunity to get an explanation of a particular term or concept . when an explanation is requested , the user is conveyed to a location in the domain modelmost apropos to the user &# 39 ; s need for an explanation . for example , as illustrated in fig4 if the diagnosis includes the term &# 34 ; ischemia &# 34 ;, by selecting the word &# 34 ; ischemia &# 34 ;, the user would be conveyed directly to the ischemia concept map . the user would then have available all of the mediating representations and other supporting devices to enable the user to interact with the domain model , to explore the domain model , to further develop an understanding of the basis for the expert system &# 39 ; s diagnosis , and / or to further understand the expert &# 39 ; s knowledge related to the domain of ischemia . thus , as seen from the foregoing description , the present invention provides a system designed to facilitate a user &# 39 ; s understanding of a domain of knowledge . from the foregoing description , it will also be clear that the principles of the present invention can be used to construct explanation tools for various types of expert systems . further , it will be apparent to those in the art that the principles of the invention can also be used to form teaching and reference tools about a topic , apart from an expert system . with the foregoing disclosure in mind , it is believed that yet additional uses for the present invention will become apparent to those of ordinary skill in the art .