ans m a =
   let o = ans' m a
   in
        case o of
            Nothing -> "no"
            Just _  -> "yes"
ans m a =
   let o = ans' m a
   in
        case o of
            Nothing -> "no"
            Just _  -> "yes"
ans m a =
   let o = ans' m a
   in
        case o of
            Nothing -> "no"
            Just _  -> "yes"