Return a format string for printing an `expr_type`
        ket/bra/ketbra/braket

def _braket_fmt(self, expr_type):
        """Return a format string for printing an `expr_type`
        ket/bra/ketbra/braket"""
        mapping = {
            'bra': {
                True: '<{label}|^({space})',
                'subscript': '<{label}|_({space})',
                False:  '<{label}|'},
            'ket': {
                True: '|{label}>^({space})',
                'subscript': '|{label}>_({space})',
                False:  '|{label}>'},
            'ketbra': {
                True: '|{label_i}><{label_j}|^({space})',
                'subscript': '|{label_i}><{label_j}|_({space})',
                False:  '|{label_i}><{label_j}|'},
            'braket': {
                True: '<{label_i}|{label_j}>^({space})',
                'subscript': '<{label_i}|{label_j}>_({space})',
                False:  '<{label_i}|{label_j}>'},
        }
        hs_setting = bool(self._settings['show_hs_label'])
        if self._settings['show_hs_label'] == 'subscript':
            hs_setting = 'subscript'
        return mapping[expr_type][hs_setting]