Lout and Index key y not allowed

[Jeff Kingston]

A bug was causing the index key y, as in "y @Index { y }", to produce an
internal error.  This was caused by a long-standing variable scoping bug
in Lout's cross-reference database.  Fixing that bug would be too hard,
but for the next release I have changed the names of the variables so
that none of them is called y, and this seems to have fixed the problem.

Jeff Kingston