code: def compose(*validators):
      
      def composed_validator(value):
          out = value
          for validator in reversed(validators):
              out = validator(out)
          return out
      composed_validator.__name__ = 'compose(%s)' % ','.join(
          val.__name__ for val in validators)
      return composed_validator
docs: Implement composition of validators. For instance
  
      >>> utf8_not_empty = compose(utf8, not_empty)