code: def f(self, y):
          
          d = self.d
          mpsi = self.psi
          z = d * y.copy()
          for i in range(len(mpsi)):
              a, b, c = mpsi[i]
              z += a * np.tanh(b * (y + c))
          return z
docs: Transform y with f using parameter vector psi
          psi = [[a,b,c]]
  
          :math:`f = (y * d) + \\sum_{terms} a * tanh(b *(y + c))`