rb1: conditionally defined rules

[Richard Boulton]

Following Tom's suggestion, I'm numbering my patches now, starting at rb1.

I submitted this one a few days ago to the wrong mailing list: it fixes and
tests a problem reported by Robert Boehne.

The reverse_conditions() function seems a little inelegant, and has the
usual combinatorial explosion problem due to using
variable_conditions_permutations(), but I think it's good enough for now,
anyway.

-- 
Richard

patch1
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