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.
