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 = {
            True: {  # use braket package
                'bra': {
                    True: r'\Bra{{{label}}}^{{({space})}}',
                    'subscript': r'\Bra{{{label}}}_{{({space})}}',
                    False:  r'\Bra{{{label}}}'},
                'ket': {
                    True: r'\Ket{{{label}}}^{{({space})}}',
                    'subscript': r'\Ket{{{label}}}_{{({space})}}',
                    False:  r'\Ket{{{label}}}'},
                'ketbra': {
                    True:
                        r'\Ket{{{label_i}}}\!\Bra{{{label_j}}}^{{({space})}}',
                    'subscript':
                        r'\Ket{{{label_i}}}\!\Bra{{{label_j}}}_{{({space})}}',
                    False: r'\Ket{{{label_i}}}\!\Bra{{{label_j}}}'},
                'braket': {
                    True: r'\Braket{{{label_i} | {label_j}}}^({space})',
                    'subscript': r'\Braket{{{label_i} | {label_j}}}_({space})',
                    False:  r'\Braket{{{label_i} | {label_j}}}'}},
            False: {  # explicit tex macros
                'bra': {
                    True:
                        r'\left\langle {label} \right\rvert^{{({space})}}',
                    'subscript':
                        r'\left\langle {label} \right\rvert^{{({space})}}',
                    False:
                        r'\left\langle {label} \right\rvert'},
                'ket': {
                    True:
                        r'\left\lvert {label} \right\rangle^{{({space})}}',
                    'subscript':
                        r'\left\lvert {label} \right\rangle_{{({space})}}',
                    False:  r'\left\lvert {label} \right\rangle'},
                'ketbra': {
                    True:
                        r'\left\lvert {label_i} \middle\rangle\!'
                        r'\middle\langle {label_j} \right\rvert^{{({space})}}',
                    'subscript':
                        r'\left\lvert {label_i} \middle\rangle\!'
                        r'\middle\langle {label_j} \right\rvert_{{({space})}}',
                    False:
                        r'\left\lvert {label_i} \middle\rangle\!'
                        r'\middle\langle {label_j} \right\rvert'},
                'braket': {
                    True:
                        r'\left\langle {label_i} \middle\vert '
                        r'{label_j} \right\rangle^{{({space})}}',
                    'subscript':
                        r'\left\langle {label_i} \middle\vert '
                        r'{label_j} \right\rangle_{{({space})}}',
                    False:
                        r'\left\langle {label_i} \middle\vert '
                        r'{label_j} \right\rangle'}}
            }
        hs_setting = bool(self._settings['show_hs_label'])
        if self._settings['show_hs_label'] == 'subscript':
            hs_setting = 'subscript'
        return mapping[self._settings['tex_use_braket']][expr_type][hs_setting]