'Return the hash of the specified format for the specified book. The
kind of hash is backend dependent, but is usually SHA-256.'

@read_api
def format_hash(self, book_id, fmt):

    try:
        name = self.fields[u'formats'].format_fname(book_id, fmt)
        path = self._field_for(u'path', book_id).replace(u'/', os.sep)
    except:
        raise NoSuchFormat((u'Record    %d    has    no    fmt:    %s' % (book_id, fmt)))
    return self.backend.format_hash(book_id, fmt, name, path)
