MAYBE
Trs:
 {f(ok(x), ok(y), ok(z)) -> ok(f(x, y, z)),
        f(x, y, mark(z)) -> mark(f(x, y, z)),
             proper(b()) -> ok(b()),
             proper(c()) -> ok(c()),
             proper(d()) -> ok(d()),
      proper(f(x, y, z)) -> f(proper(x), proper(y), proper(z)),
             active(d()) -> m(b()),
             active(d()) -> mark(c()),
  active(f(b(), c(), x)) -> mark(f(x, x, x)),
      active(f(x, y, z)) -> f(x, y, active(z)),
            top(mark(x)) -> top(proper(x)),
              top(ok(x)) -> top(active(x))}
 Comment:
  We consider a duplicating trs.
  FAIL:
   Open