MAYBE

Problem:
 active(f(X)) -> mark(if(X,c(),f(true())))
 active(if(true(),X,Y)) -> mark(X)
 active(if(false(),X,Y)) -> mark(Y)
 mark(f(X)) -> active(f(mark(X)))
 mark(if(X1,X2,X3)) -> active(if(mark(X1),mark(X2),X3))
 mark(c()) -> active(c())
 mark(true()) -> active(true())
 mark(false()) -> active(false())
 f(mark(X)) -> f(X)
 f(active(X)) -> f(X)
 if(mark(X1),X2,X3) -> if(X1,X2,X3)
 if(X1,mark(X2),X3) -> if(X1,X2,X3)
 if(X1,X2,mark(X3)) -> if(X1,X2,X3)
 if(active(X1),X2,X3) -> if(X1,X2,X3)
 if(X1,active(X2),X3) -> if(X1,X2,X3)
 if(X1,X2,active(X3)) -> if(X1,X2,X3)

Proof:
 Open