```ucm:hide
scratch/main> builtins.merge
```

```unison
structural ability X t where
  x : t -> a -> a

structural ability Abort where
  abort : a
```

```ucm
scratch/main> add
```

This code should not type check. The match on X.x ought to introduce a
skolem variable `a` such that `c : a` and the continuation has type
`a ->{X} b`. Thus, `handle c with h : Optional a`, which is not the
correct result type.

```unison:error
h0 : Request {X t} b -> Optional b
h0 req = match req with
  { X.x _ c -> _ } -> handle c with h0
  { d } -> Some d
```

This code should not check because `t` does not match `b`.

```unison:error
h1 : Request {X t} b -> Optional b
h1 req = match req with
  { X.x t _ -> _ } -> handle t with h1
  { d } -> Some d
```

This code should not check for reasons similar to the first example,
but with the continuation rather than a parameter.

```unison:error
h2 : Request {Abort} r -> r
h2 req = match req with
  { Abort.abort -> k } -> handle k 5 with h2
  { r } -> r
```

This should work fine.

```unison
h3 : Request {X b, Abort} b -> Optional b
h3 = cases
  { r } -> Some r
  { Abort.abort -> _ } -> None
  { X.x b _ -> _ } -> Some b
```