Make the loop-breaking algorithm a bit more liberal, where RULES are involved

Make the loop-breaking algorithm a bit more liberal, where RULES are involved

This is another gloss on the now-quite-subtle and heavily-documented algorithm
for choosing loop breakers.

This fix, provoked by Roman's NDP library, makes sure that when we are choosing
a loop breaker we only take into account variables free on the *rhs* of a rule
not the *lhs*.

Most of the new lines are comments!