MAYBE
* Step 1: Failure MAYBE
  + Considered Problem:
      - Strict TRS:
          active(b()) -> mark(a())
          active(f(X,X)) -> mark(f(a(),b()))
          active(f(X1,X2)) -> f(active(X1),X2)
          f(mark(X1),X2) -> mark(f(X1,X2))
          f(ok(X1),ok(X2)) -> ok(f(X1,X2))
          proper(a()) -> ok(a())
          proper(b()) -> ok(b())
          proper(f(X1,X2)) -> f(proper(X1),proper(X2))
          top(mark(X)) -> top(proper(X))
          top(ok(X)) -> top(active(X))
      - Signature:
          {active/1,f/2,proper/1,top/1} / {a/0,b/0,mark/1,ok/1}
      - Obligation:
          innermost runtime complexity wrt. defined symbols {active,f,proper,top} and constructors {a,b,mark,ok}
  + Applied Processor:
      NaturalPI {shape = Mixed 3, restrict = NoRestrict, uargs = UArgs, urules = URules, selector = Nothing}
  + Details:
      Incompatible
MAYBE