def read_meta_line(self, f):
        """Read one line from a file format header and return a (name, value)
        tuple, where name is lowercased. Returns ('', '') if the next line is
        blank. Raises ParsingError if next line in f is not a valid header
        line."""
        line = f.readline()
        if isinstance(line, bytes):
            line = line.decode('utf-8-sig')
        name, value = '', ''
        if line.isspace():
            return (name, value)
        if line.find('=') > 0:
            name, value = line.split('=', 1)
            name = name.rstrip().lower()
            value = value.strip()
        name = '' if any([c.isspace() for c in name]) else name
        if not name:
            raise ParsingError("Invalid file header line")
        return (name, value)