[Enhancement?] Reduce the searching size of shading-relevant puzzles

[T0nyX1ang]

In a common shading puzzle, the shading rule should be:

    { black(R, C) } :- grid(R, C).
    black(r1, c1).
    not black(r2, c2).

But the shading space could be reduced by:

    { black(R, C) } :- grid(R, C), not clue(R, C).
    clue(R, C).

We are unsure whether the second approach is better.

[T0nyX1ang]

Pro: possibly save time.
Con: cases for no clues should be specially judged.

A middle way: list all possible excluded cells.

[T0nyX1ang]

After a discussion, we decide to close this issue.