docs: def _forward_mode(self, *args):
          """Forward mode differentiation for a sum"""
          # (f+g)(x) = f(x) + g(x)
          f_val, f_diff = self.f._forward_mode(*args)
          g_val, g_diff = self.g._forward_mode(*args)
          # The function value and derivative is the sum of f and g
          val = f_val + g_val
          diff = f_diff + g_diff
          return val, diff