code: def _validate(item):
          
          if not isinstance(item, tuple) or len(item) != 2:
              raise ValueError('Linked items must be instance/prop-name tuple')
          if not isinstance(item[0], tuple(LINK_OBSERVERS)):
              raise ValueError('Only {} instances may be linked'.format(
                  ', '.join([link_cls.__name__ for link_cls in LINK_OBSERVERS])
              ))
          if not isinstance(item[1], string_types):
              raise ValueError('Properties must be specified as string names')
          if not hasattr(item[0], item[1]):
              raise ValueError('Invalid property {} for {} instance'.format(
                  item[1], item[0].__class__.__name__
              ))
docs: Validate (instance, prop name) tuple