The identifier (symbol) that is used when printing the annihilation
        operator. This is identical to the identifier of :class:`Create`. A
        custom identifier for both :class:`Destroy` and :class:`Create` can be
        set through the `local_identifiers` parameter of the associated Hilbert
        space::

            >>> hs_custom = LocalSpace(0, local_identifiers={'Destroy': 'b'})
            >>> Create(hs=hs_custom).identifier
            'b'
            >>> Destroy(hs=hs_custom).identifier
            'b'

def identifier(self):
        """The identifier (symbol) that is used when printing the annihilation
        operator. This is identical to the identifier of :class:`Create`. A
        custom identifier for both :class:`Destroy` and :class:`Create` can be
        set through the `local_identifiers` parameter of the associated Hilbert
        space::

            >>> hs_custom = LocalSpace(0, local_identifiers={'Destroy': 'b'})
            >>> Create(hs=hs_custom).identifier
            'b'
            >>> Destroy(hs=hs_custom).identifier
            'b'
        """
        identifier = self._hs._local_identifiers.get(
            self.__class__.__name__, self._hs._local_identifiers.get(
                'Create', self._identifier))
        if not self._rx_identifier.match(identifier):
            raise ValueError(
                "identifier '%s' does not match pattern '%s'"
                % (identifier, self._rx_identifier.pattern))
        return identifier