[rfwkfqbs] No duals

Test whether the solution to a mate in N chess problem has no duals.  I think this means, on every path of most resistance (there may be multiple such paths because the defender could defend multiple ways) the attacker has a unique move to complete mate in N.  If the defender makes suboptimal moves, then the attacker's best moves need not be unique.

Perhaps relax the constraint toward the end when it is mate in 1 or 2.  Or some sort of clearly winning position in a "play and win" problem.