Representation of a leading piece of a hierarchical reference to something, perhaps with associated indices.
This is a product type introduced by defprod.
A vl-hidindex only makes sense in the context of a larger vl-hidexpr. Consider an hierarchical indexing expression like
cat . (dog [3][2][1]) . elf)
The dog[3][2][1] part of this will be represented by a vl-hidindex whose name is dog and whose indices are the expressions for 3, 2, and 1.
The cat part of this will be represented by a vl-hidindex whose name is cat and whose indices are nil.
