code: def contains_only(self, *items):
          
          if len(items) == 0:
              raise ValueError('one or more args must be given')
          else:
              extra = []
              for i in self.val:
                  if i not in items:
                      extra.append(i)
              if extra:
                  self._err('Expected <%s> to contain only %s, but did contain %s.' % (self.val, self._fmt_items(items), self._fmt_items(extra)))
  
              missing = []
              for i in items:
                  if i not in self.val:
                      missing.append(i)
              if missing:
                  self._err('Expected <%s> to contain only %s, but did not contain %s.' % (self.val, self._fmt_items(items), self._fmt_items(missing)))
          return self
docs: Asserts that val contains only the given item or items.