code: def _call(self, x, out=None):
          
          if out is None:
              return self.prod_op(x)[0]
          else:
              wrapped_out = self.prod_op.range.element([out], cast=False)
              pspace_result = self.prod_op(x, out=wrapped_out)
              return pspace_result[0]
docs: Apply operators to ``x`` and sum.