def split_on_delimiter(comments, delimiter):
    """Given the comments string, split the "original" comments from 
    any existing structured comments on the delimiter. Return the 
    original comments and the existing structured comments parts.
    """
    # split on the delimiter
    parts = [s.strip() for s in comments.split(delimiter, 1)]
    if len(parts) == 1:
        return None, parts[0]
    else:
        return tuple(parts)