Comment by apetersson

_ = (lambda _, __, ___: (lambda _, __, ___, ____: {_[___](____)[_(__)](_(___, ____)) for ___ in _}[_](__, _(___, __))) ( (lambda _: _ % 2 and _ // 2 or _ * 5 + 1), (lambda _: _ << 1), 2, 1 ) )[(lambda _, __: _ << __), 5, 2]

Guess what that is doing