MAYBE

Problem:
 a__f(a(),b(),X) -> a__f(mark(X),X,mark(X))
 a__c() -> a()
 a__c() -> b()
 mark(f(X1,X2,X3)) -> a__f(mark(X1),X2,mark(X3))
 mark(c()) -> a__c()
 mark(a()) -> a()
 mark(b()) -> b()
 a__f(X1,X2,X3) -> f(X1,X2,X3)
 a__c() -> c()

Proof:
 Open