18. The computer system of claim 17 , wherein the documents are displayed by the display program module as nodes of a tree structure having links and nodes in which similarity of documents is indicated by proximity of nodes to each other and by a length of links connecting the nodes to a common vertex.
18. The computer system of claim 17 , wherein the documents are displayed by the display program module as nodes of a tree structure having links and nodes in which similarity of documents is indicated by proximity of nodes to each other and by a length of links connecting the nodes to a common vertex. 19. The computer system of claim 18 , wherein the documents are displayed by the display program module as nodes of a tree structure that represents documents stored under control of at least one third-tier program module.