def append(self, *exprs):
self._assert_supports_contents()<EOL>self._contents.extend(exprs)<EOL>
Add contents to the expression.

        :param Union[TexExpr,str] exprs: List of contents to add

        >>> expr = TexExpr('textbf', ('hello',))
        >>> expr
        TexExpr('textbf', ['hello'])
        >>> expr.append('world')
        >>> expr
        TexExpr('textbf', ['hello', 'world'])