catchFail
  <try>
  | push (\u1 -> (\u2 -> (\u3 -> (\u4 -> 'a' : ('b' : ('c' : ('d' : Pure)))))))
  | read ('a' ==)
  | lift (\u1 -> (\u2 -> u1 u2))
  | read ('b' ==)
  | lift (\u1 -> (\u2 -> u1 u2))
  | read ('c' ==)
  | lift (\u1 -> (\u2 -> u1 u2))
  | read ('d' ==)
  | lift (\u1 -> (\u2 -> u1 u2))
  | popFail
  | ret
  <handler>
    loadInput
    fail