graph reduction

   A technique invented by Chris Wadsworth where an expression is
   represented as a directed graph (usually drawn as an
   inverted tree).  Each node represents a function call and its
   subtrees represent the arguments to that function.  Subtrees
   are replaced by the expansion or value of the expression they
   represent.  This is repeated until the tree has been reduced
   to a value with no more function calls (a normal form).

   In contrast to string reduction, graph reduction has the
   advantage that common subexpressions are represented as
   pointers to a single instance of the expression which is only
   reduced once.  It is the most commonly used technique for
   implementing lazy evaluation.