Problem:
 f(X) -> if(X,c(),f(true()))
 if(true(),X,Y) -> X
 if(false(),X,Y) -> Y

Proof:
 Containment Processor: loop length: 1
                        terms:
                         f(X)
                        context: if(X,c(),[])
                        substitution:
                         X -> true()
  Qed