def parse_hl_lines(expr):
    """Support our syntax for emphasizing certain lines of code.

    expr should be like '1 2' to emphasize lines 1 and 2 of a code block.
    Returns a list of ints, the line numbers to emphasize.
    """
    if not expr:
        return []

    try:
        return list(map(int, expr.split()))
    except ValueError:  # pragma: no cover
        return []