Union closure of L(nrNIDPDAwtl)

Determine whether the language family L(nrNIDPDAwtl), consisting of all languages accepted by non-returning nondeterministic input-driven pushdown automata with translucent input letters, is closed under union when automata may have incompatible signatures, without assuming signature compatibility.

Background

For the non-returning nondeterministic model with translucent letters, the paper establishes non-closure under complementation but leaves union closure unresolved in the general case.

While union closure holds when signatures are compatible, the open question concerns the general case without that restriction.