input: Write a python function to combine elements from two lists by summing them pairwise.
  assert foo([1, 2, 3], [4, 5, 6]) == [5, 6, 7, 6, 7, 8, 7, 8, 9]
  assert foo([0, -1], [1, 1]) == [1, 1, 0, 0]
  assert foo([10], [10, 20, 30]) == [20, 30, 40]
output: def foo(list1, list2):
      result = []
      for i in list1:
          for j in list2:
              result.append(i + j)
      return result