Convert a BOOST_METAPARSE_STRING mode document into one with
        this mode

def convert_from(self, base):
        """Convert a BOOST_METAPARSE_STRING mode document into one with
        this mode"""
        if self.identifier == 'bmp':
            return base
        elif self.identifier == 'man':
            result = []
            prefix = 'BOOST_METAPARSE_STRING("'
            while True:
                bmp_at = base.find(prefix)
                if bmp_at == -1:
                    return ''.join(result) + base
                else:
                    result.append(
                        base[0:bmp_at] + '::boost::metaparse::string<'
                    )
                    new_base = ''
                    was_backslash = False
                    comma = ''
                    for i in xrange(bmp_at + len(prefix), len(base)):
                        if was_backslash:
                            result.append(
                                '{0}\'\\{1}\''.format(comma, base[i])
                            )
                            was_backslash = False
                            comma = ','
                        elif base[i] == '"':
                            new_base = base[i+2:]
                            break
                        elif base[i] == '\\':
                            was_backslash = True
                        else:
                            result.append('{0}\'{1}\''.format(comma, base[i]))
                            comma = ','
                    base = new_base
                    result.append('>')