Extract fields in a parse expression schema.

        :param schema: Parse expression schema/format to use (as string).
        :return: Generator for fields in schema (as Field objects).

def extract_fields(cls, schema):
        """Extract fields in a parse expression schema.

        :param schema: Parse expression schema/format to use (as string).
        :return: Generator for fields in schema (as Field objects).
        """
        # -- BASED-ON: parse.Parser._generate_expression()
        for part in parse.PARSE_RE.split(schema):
            if not part or part == '{{' or part == '}}':
                continue
            elif part[0] == '{':
                # this will be a braces-delimited field to handle
                yield cls.parse(part)