In our descriptions of data structures we have a finite fixed set of named directions, which correspond to pointers leaving the nodes of the graph of that structure. A subset of these directions are generating directions or generators, these links form a tree skeleton for the graph. The list of generators from the root node to a node as the description of that node. This list of generating directions is called the path name. A word is a string of directions, and differs in that we append words to path names to produce new path names.