code: def positiveints(value):
      
      ints = integers(value)
      for val in ints:
          if val < 0:
              raise ValueError('%d is negative in %r' % (val, value))
      return ints
docs: >>> positiveints('1, -1')
      Traceback (most recent call last):
         ...
      ValueError: -1 is negative in '1, -1'