Explanation: You can ❰merge❱ the alternatives of a union using a record with one
handler per alternative, like this:


    ┌─────────────────────────────────────────────────────────────────────┐
    │     let union    = < Left = +2 | Right : Bool >                     │
    │ in  let handlers = { Left = Natural/even, Right = λ(x : Bool) → x } │
    │ in  merge handlers union : Bool                                     │
    └─────────────────────────────────────────────────────────────────────┘


... but you must provide exactly one handler per alternative in the union.  You
cannot omit any handlers

For example, the following expression is $_NOT valid:


                                              Invalid: Missing ❰Right❱ handler
                                              ⇩
    ┌─────────────────────────────────────────────────┐
    │     let handlers = { Left = Natural/even }      │
    │ in  let union    = < Left = +2 | Right : Bool > │
    │ in  merge handlers union : Bool                 │
    └─────────────────────────────────────────────────┘


Note that you need to provide handlers for other alternatives even if those
alternatives are never used

You need to supply the following handlers:

↳ $txt0