(let f := fun x => pure 0;
    f ()).run
  0 : Id (Nat × Nat)
((let x := fun x => pure 0;
      x ()).run
    0).run : Nat × Nat
(let x := fun x => pure [0];
    x ()).run
  0 : Id (List Nat × Nat)
(let x := fun x => pure Syntax.missing;
  x ()).run : Syntax