code: def build_y(self):
          
          if not self.n:
              return
          self.y1 = mul(self.u, self.g1 + self.b1 * 1j)
          self.y2 = mul(self.u, self.g2 + self.b2 * 1j)
          self.y12 = div(self.u, self.r + self.x * 1j)
          self.m = polar(self.tap, self.phi * deg2rad)
          self.m2 = abs(self.m)**2
          self.mconj = conj(self.m)
  
          # build self and mutual admittances into Y
          self.Y = spmatrix(
              div(self.y12 + self.y1, self.m2), self.a1, self.a1,
              (self.nb, self.nb), 'z')
          self.Y -= spmatrix(
              div(self.y12, self.mconj), self.a1, self.a2, (self.nb, self.nb),
              'z')
          self.Y -= spmatrix(
              div(self.y12, self.m), self.a2, self.a1, (self.nb, self.nb), 'z')
          self.Y += spmatrix(self.y12 + self.y2, self.a2, self.a2,
                             (self.nb, self.nb), 'z')
docs: Build transmission line admittance matrix into self.Y