def get_comment_header(line):
    """Return string with leading whitespace and '#' from line or ''.

    A null return indicates that the line is not a comment line. A non-
    null return, such as '    #', will be used to find the other lines of
    a comment block with the same  indent.
    """
    m = re.match(r"^([ \t]*#*)", line)
    if m is None: return ""
    return m.group(1)