docs: def get_slope(self):
          """Return the slope m of this line segment."""
          # y1 = m*x1 + t
          # y2 = m*x2 + t => y1-y2 = m*(x1-x2) <=> m = (y1-y2)/(x1-x2)
          return ((self.p1.y-self.p2.y) / (self.p1.x-self.p2.x))