code: def _series_expand_combine_prod(c1, c2, order):
      
      from qnet.algebra.core.scalar_algebra import Zero
      res = []
      c1 = list(c1)
      c2 = list(c2)
      for n in range(order + 1):
          summands = []
          for k in range(n + 1):
              if c1[k].is_zero or c2[n-k].is_zero:
                  summands.append(Zero)
              else:
                  summands.append(c1[k] * c2[n - k])
          sum = summands[0]
          for summand in summands[1:]:
              if summand != 0:
                  sum += summand
          res.append(sum)
      return tuple(res)
docs: Given the result of the ``c1._series_expand(...)`` and
      ``c2._series_expand(...)``, construct the result of
      ``(c1*c2)._series_expand(...)``