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)
          # The function value
          val = self.func(X)
          # The derivative
          diff = self.deriv(X) * dX
          return (val, diff)