code: def addevensubodd(operator, operand):
      
      try:
          for i, x in enumerate(operand):
              if x % 2:
                  operand[i] = -x
          return operand
      except TypeError:
          if operand % 2:
              return -operand
          return operand
docs: Add even numbers, subtract odd ones. See http://1w6.org/w6