def _removeHeaderTag(header, tag):
    """Removes a tag from the beginning of a header string.

    :param header: str
    :param tag: str
    :returns: (str, bool), header without the tag and a bool that indicates
        wheter the tag was present.
    """
    if header.startswith(tag):
        tagPresent = True
        header = header[len(tag):]
    else:
        tagPresent = False
    return header, tagPresent