code: def is_in(self, *items):
          
          if len(items) == 0:
              raise ValueError('one or more args must be given')
          else:
              for i in items:
                  if self.val == i:
                      return self
          self._err('Expected <%s> to be in %s, but was not.' % (self.val, self._fmt_items(items)))
docs: Asserts that val is equal to one of the given items.