MAYBE

Problem:
 active(f(b(),X,c())) -> mark(f(X,c(),X))
 active(c()) -> mark(b())
 active(f(X1,X2,X3)) -> f(X1,active(X2),X3)
 f(X1,mark(X2),X3) -> mark(f(X1,X2,X3))
 proper(f(X1,X2,X3)) -> f(proper(X1),proper(X2),proper(X3))
 proper(b()) -> ok(b())
 proper(c()) -> ok(c())
 f(ok(X1),ok(X2),ok(X3)) -> ok(f(X1,X2,X3))
 top(mark(X)) -> top(proper(X))
 top(ok(X)) -> top(active(X))

Proof:
 Open