def isheader(self, line):
i = line.find('<STR_LIT::>')<EOL>if i > -<NUM_LIT:1>:<EOL><INDENT>return line[:i].lower()<EOL><DEDENT>return None<EOL>
Determine whether a given line is a legal header.

        This method should return the header name, suitably canonicalized.
        You may override this method in order to use Message parsing on tagged
        data in RFC 2822-like formats with special header formats.