Represents a single frame in the LP interpreter's environment stack. The
environment stack represents the AND part of the search tree - it is a sequence
of nested predicate calls.
This is used in the inner loop of the interpreter and so is a pure data structure
not an abstract data type and assumes privileged access to the interpreter state.
author: Dave Reynolds version: $Revision: 1.7 $ on $Date: 2008/01/02 12:06:16 $