Error: the pattern
  left(x)
is supposed to have type
  List(ℤ),
but instead it has a sum type.
https://disco-lang.readthedocs.io/en/latest/reference/pattern-type.html
Error: the pattern
  (x1, y)
is supposed to have type
  ℕ,
but instead it has a product type.
https://disco-lang.readthedocs.io/en/latest/reference/pattern-type.html
Error: the shape of two types does not match.
https://disco-lang.readthedocs.io/en/latest/reference/shape-mismatch.html