catchFail
  <try>
  | push (\x0 -> (\x1 -> x0)) 'a'
  | read
  | lift (\x0 -> (\x1 -> x0 x1))
  | popFail
  | ret
  <handler>
    pushInput
    lift Pure
    choices [id]
      <branch>
      | push (\x0 -> (\x1 -> x0)) 'b'
      | read
      | lift (\x0 -> (\x1 -> x0 x1))
      | ret
      <default>
        fail