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                                     │
    └─────────────────────────────────────────────────────────────────────┘


... as long as the input type of each handler function matches the type of the
corresponding alternative:


    ┌───────────────────────────────────────────────────────────┐
    │ union    : < Left : Natural       | Right : Bool        > │
    └───────────────────────────────────────────────────────────┘
                          ⇧                       ⇧
                   These must match        These must match
                          ⇩                       ⇩
    ┌───────────────────────────────────────────────────────────┐
    │ handlers : { Left : Natural → Bool, Right : Bool → Bool } │
    └───────────────────────────────────────────────────────────┘


For example, the following expression is $_NOT valid:


      Invalid: Doesn't match the type of the ❰Right❱ alternative
                                                               ⇩
    ┌──────────────────────────────────────────────────────────────────────┐
    │     let handlers = { Left = Natural/even | Right = λ(x : Text) → x } │
    │ in  let union    = < Left = +2 | Right : Bool >                      │
    │ in  merge handlers union : Bool                                      │
    └──────────────────────────────────────────────────────────────────────┘


Your handler for the following alternative:

↳ $txt0

... needs to accept an input value of type:

↳ $txt1

... but actually accepts an input value of a different type:

↳ $txt2