def listMinus(x, y, delete=False):
    """If delete = True, removes all things in y that are in x. e.g.
    x = [1,2,3,4], y = [2,4] at the end x becomes [1,3], y remains the same.
    If delete = False, returns what x would have been while preserving x."""
    if delete == True:
        to_be_diminished = x
    else:
        to_be_diminished = x.copy()

    for thing in y:
        try:
            to_be_diminished.remove(thing)
        except ValueError:
            pass

    if delete == False:
        return(to_be_diminished)