Like term_expansion/2, function_expansion/3 provides for macro
expansion of Prolog source code. In this case, by expanding
Term which is nested inside a parent term. Term is replaced with
Replacement. Guard is placed as a conjunction before the parent
term. Guard typically binds Replacement in some useful fashion.
For example, a function macro which doubles its argument might
user:function_expansion(double(X), Y, Y is 2*X).
V = 9,
format('~p times 2 is ~p~n', [V, double(V)]).
V = 9,
A is 2*V,
format('~p times 2 is ~p~n', [V, A]).
Mathematical constants might be implemented like
user:function_expansion(pi, 3.14159, true).