def lchop(s, sub):
    """Chop ``sub`` off the front of ``s`` if present.

    >>> lchop("##This is a comment.##", "##")
    'This is a comment.##'

    The difference between ``lchop`` and ``s.lstrip`` is that ``lchop`` strips
    only the exact prefix, while ``s.lstrip`` treats the argument as a set of
    leading characters to delete regardless of order.
    """
    if s.startswith(sub):
        s = s[len(sub):]
    return s