Picking NaturalTransformation component crashes

[shingarov]

Trying to check the following program:

⟦measure len : list(''a) => int⟧

type list(''a) =
  | Nil                        => [v| v⋅len === 0 ]
  | Cons (x:''a, xs:list(''a)) => [v| v⋅len === (xs⋅len + 1) ]
  ;

⟦val check: list(''a)[v| v⋅len === 0 ] => int⟧
let check = (x) => {
	0
};

crashes because the natural transformation len has no components.