docs: def _forward_mode(self, *args):
          """Forward mode differentiation for a constant"""
          # Evaluate inner function self.f
          X: np.ndarray
          dX: np.ndarray
          X, dX = self.f._forward_mode(*args)
          Y: np.ndarray
          dY: np.ndarray
          Y, dY = self.g._forward_mode(*args)
          # The function value
          val = self.func(X,Y)
          # The derivative (user is responsible for chain rule, etc.)
          diff = self.deriv(X,Y)
          return (val, diff)