code: def coupling(self, source_y, target_y, weight):
          
          v_pyramidal = source_y[1] - source_y[2]
          return (np.array([0, 0, 0, 0, 0, 1.0, 0, 0]) *
                  (weight*self.g1*self.He2*self.ke2*self.S(v_pyramidal)))
docs: How to couple the output of one node to the input of another.
          Args:
            source_y (array of shape (8,)): state of the source node
            target_y (array of shape (8,)): state of the target node
            weight (float): the connection strength
          Returns:
            input (array of shape (8,)): value to drive each variable of the
              target node.