code: def horizontal_angle(C, P):
          
          S = Point(P - C).unit                   # unit vector 'C' -> 'P'
          alfa = math.asin(abs(S.y))              # absolute angle from horizontal
          if S.x < 0:                             # make arcsin result unique
              if S.y <= 0:                        # bottom-left
                  alfa = -(math.pi - alfa)
              else:                               # top-left
                  alfa = math.pi - alfa
          else:
              if S.y >= 0:                        # top-right
                  pass
              else:                               # bottom-right
                  alfa = - alfa
          return alfa
docs: Return the angle to the horizontal for the connection from C to P.
          This uses the arcus sine function and resolves its inherent ambiguity by
          looking up in which quadrant vector S = P - C is located.