The key type of a Union map: an opaque non-negative integer string ("0", "1", …) labelling one
branch. The key only names the branch; which variant that branch retrieves is fixed by matching it against the
property's declared variants, never by the key value, so renaming or reordering keys leaves an existing template
valid. The `${number}` key space is disjoint from the Binding / Identifier key spaces used by
Template and Projection, keeping form discrimination structural and unambiguous.
Union branch key.
The key type of a Union map: an opaque non-negative integer string (
"0","1", …) labelling one branch. The key only names the branch; which variant that branch retrieves is fixed by matching it against the property's declared variants, never by the key value, so renaming or reordering keys leaves an existing template valid. The`${number}`key space is disjoint from the Binding / Identifier key spaces used by Template and Projection, keeping form discrimination structural and unambiguous.