Get a string describing a kind.

def str_kind(self, kind):
        """Get a string describing a kind."""
        if len(kind) == 0:
            return 'Nothing'
        elif len(kind) == 1:
            return kind[0].__name__
        elif len(kind) == 2:
            return kind[0].__name__ + ' or ' + kind[1].__name__
        else:
            return 'one of {' + ', '.join(t.__name__ for t in kind) + '}'