lets
` <|>
  + <*>
  | + <*>
  | | + pure (\u1 -> (\u2 -> u1))
  | | ` pure 'a'
  | ` satisfy
  ` <*>
    + <*>
    | + pure (\u1 -> (\u2 -> u1))
    | ` pure 'b'
    ` satisfy