I am COMPLETELY GORMLESS. The Solo grid generator, when eliminating

I am COMPLETELY GORMLESS. The Solo grid generator, when eliminating
clues from a filled grid, was using the algorithm

 - loop over the whole grid looking for a clue (or symmetry group of
   clues) which can be safely removed
 - remove it
 - loop over the whole grid again, and so on.

This was due to my vague feeling that removing one clue might affect
whether another can be removed. Of course this can happen - two
clues can be alternative ways of deducing the same vital fact so
that removing one makes the other necessary - but what _can't_
happen is for removing one clue to make another _become_ removable,
since you can only do that by _adding_ information. In other words,
after testing a clue and determining that it can't be removed, you
never need to test it again. Thus, a much simpler algorithm is

 - loop over the possible clues (or symmetry groups) _once_, in a
   random order
 - for each clue (group), if it is removable, remove it.

This still guarantees to leave the grid in a state where no further
clues can be removed, but it greatly cuts down puzzle generation
time and also simplifies the code. I am a fool for not having
spotted this in three and a half months!

[originally from svn r6160]