fact: Definition queens_explor n : M _ := do s <- get; do rs <- perms (map Z_of_nat (iota 0 n)); put empty >> (do ok <- safe2 (place n rs) ; (guard ok >> put s)) >> Ret rs.
imports: ZArith mathcomp(all_ssreflect) infotheo(ssrZ) monae_lib hierarchy monad_lib fail_lib state_lib