Schemes:
   f(a,b)
   g(c,d)
   r(e,f)

Facts:
   f('1','2').
   f('4','3').
   g('3','2').
   r('3','5').

Rules:
   r(E,F):-f(E,F).
   g(C,D):-f(C,X),r(X,D).

Queries:
   g('4',B)?
   r(E,'3')?
   f(A,B)?
   g(A,B)?
   r(A,B)?