Function cross_negative

src/fields.cpp:398–402

}

static inline bool cross_negative(direction a, direction b) {
  if (a >= R) a = direction(a - 3);
  if (b >= R) b = direction(b - 3);
  return ((3 + b - a) % 3) == 2;
}

static inline direction cross(direction a, direction b) {
  if (a == b) meep::abort("bug - cross expects different directions");