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 output type of each handler function matches the declared type
of the result:


    ┌───────────────────────────────────────────────────────────┐
    │ handlers : { Left : Natural → Bool, Right : Bool → Bool } │
    └───────────────────────────────────────────────────────────┘
                                    ⇧                    ⇧
                                    These output types ...

                             ... must match the declared type of the ❰merge❱
                             ⇩
    ┌─────────────────────────────┐
    │ merge handlers union : Bool │
    └─────────────────────────────┘


For example, the following expression is $_NOT valid:


    ┌──────────────────────────────────────────────────────────────────────┐
    │     let union    = < Left = +2 | Right : Bool >                     │
    │ in  let handlers = { Left = Natural/even, Right = λ(x : Bool) → x } │
    │ in  merge handlers union : Text                                      │
    └──────────────────────────────────────────────────────────────────────┘
                                 ⇧
                                 Invalid: Doesn't match output of either handler


Your handler for the following alternative:

↳ $txt0

... needs to return an output value of type:

↳ $txt1

... but actually returns an output value of a different type:

↳ $txt2