Tektronix Neptune is one hypertext system that has been particularly designed as an open, layered architecture. Neptune strongly spearates the front end, a Smalltalk-based user interface, form the back end, a transaction-based server called the Hypertext Abstract Machine (HAM). The HAM is a generic hypertext model which provides operations for creating, modifying, and acessing nodes and links. It maintains a complete verison history of each node in the hyeprdocument, and provides rapid access to any version of a hyperdocument. It rpovides distributed access over a computer network, synchronization for multiuser access, a complex network versioning scheme, and transaction-based crash recovery. The interface layer provides serveral browsers: A graph browser provides a pictorial view of a subgraph of nodes and links; a document browser supports the browsing of hierarchical structures of nodes and links; and a node browser accesses an individual node in a hyper-document. Other browsers include attribute browsers, version browsers, node differences browsers, and demon browsers. (Conklin, 1987)