Bug in module scoping of nested substructures with `with'

Hi,

Suppose more than one module defines a structure with the same name and
arity (say foo/2), and exactly one of these modules is imported into another
module M.  If foo/2 is named as a substructure of a structure in M, and then
`with' is used to access foo/2 or any of its substructures, then both
definitions of foo/2 are visible.

Attached is an example where a perfectly legal structure usage results in
the wrong answer because the wrong definition of foo/2 was selected by
`with'.

I intend to commit a fix and a test for this shortly.

Cheers,
Warwick