def matchBytes(self, bytes):
        """Look for a sequence of bytes at the start of a string. If the bytes
        are found return True and advance the position to the byte after the
        match. Otherwise return False and leave the position alone"""
        p = self.position
        data = self[p:p + len(bytes)]
        rv = data.startswith(bytes)
        if rv:
            self.position += len(bytes)
        return rv
Look for a sequence of bytes at the start of a string. If the bytes
        are found return True and advance the position to the byte after the
        match. Otherwise return False and leave the position alone