Labels one alternative of a union so its branches can be told apart. The label carries no meaning of its own: the
variants a branch retrieves are fixed by its shape, never by its label, so any set of distinct labels serves equally
well.
Important
A branch key MUST be a canonical non-negative integer string with no leading zeros, such as "0" or "42". The
type also admits forms such as "01", "-1" or "1.5" only because TypeScript cannot narrow a template literal
to canonical integers; isBranch rejects them at runtime.
Union branch key.
Labels one alternative of a union so its branches can be told apart. The label carries no meaning of its own: the variants a branch retrieves are fixed by its shape, never by its label, so any set of distinct labels serves equally well.
A branch key MUST be a canonical non-negative integer string with no leading zeros, such as
"0"or"42". The type also admits forms such as"01","-1"or"1.5"only because TypeScript cannot narrow a template literal to canonical integers; isBranch rejects them at runtime.