[developers] generation/224: additional generator trigger rule facility

[Ann Copestake]

It's certainly partly a matter of caching things out, though the lex-id implies
lex-id idea, which is one way of characterising what I'm proposing, is
potentially more restrictive than FS -> lex-id.  However, if the FSs are all
lexical entries (albeit with instantiated MRSs), I suspect there's little
practical difference.

just to clarify though, I wasn't proposing that this would only work with the
postgres database - it would just be another thing computed as part of the
`index for generation'.  I'm committed to keeping everything working without
the DB since making people install postgres before they can use the LKB is not
an option (though I do think people with large lexicons should be using the 
lexdb)

Ann