Automatic generation of temporary identifiers for racket syntax templates,
based on subscripts indices on the identifiers (e.g. generate yᵢ … from xᵢ …).
Note that the syntax pattern variables must be matched with one of the patched
forms from stxparse-info/parse or stxparse-info/case, instead of the
syntax pattern-matching forms from syntax/parse or `racket/base,
respectively.