def line_of_sight?(x1, y1, x2, y2)
      !line_of_sight_blocked_at(x1, y1, x2, y2)
    end
def line_of_sight?(x1, y1, x2, y2)
      !line_of_sight_blocked_at(x1, y1, x2, y2)
    end
Does the point x1, x2 have line of sight to x2, y2 (that is, no solid in the way).