+ *
+ * A positive or zero value indicates what it sounds as if it
+ * should; -1 indicates that this square _cannot_ be part of
+ * this rectangle; and -2 indicates that it _definitely_ is
+ * (which is distinct from 1, because one might very well know
+ * that _if_ square S is part of rectangle R then it must be
+ * because R is placed in a certain position without knowing
+ * that it definitely _is_).