MAYBE

Problem:
 active(f(a(),X,X)) -> mark(f(X,b(),b()))
 active(b()) -> mark(a())
 mark(f(X1,X2,X3)) -> active(f(X1,mark(X2),X3))
 mark(a()) -> active(a())
 mark(b()) -> active(b())
 f(mark(X1),X2,X3) -> f(X1,X2,X3)
 f(X1,mark(X2),X3) -> f(X1,X2,X3)
 f(X1,X2,mark(X3)) -> f(X1,X2,X3)
 f(active(X1),X2,X3) -> f(X1,X2,X3)
 f(X1,active(X2),X3) -> f(X1,X2,X3)
 f(X1,X2,active(X3)) -> f(X1,X2,X3)

Proof:
 Open