def _unpackXY(x, y):
    """If x is a sequence and y is None, returns x[0], y[0]. Else, returns x, y.

    On functions that receive a pair of x,y coordinates, they can be passed as
    separate arguments, or as a single two-element sequence.
    """
    if isinstance(x, str):
        # x parameter is the string of an image filename to find and click on:
        x, y = center(locateOnScreen(x))

    elif isinstance(x, collectionsSequence):
        if len(x) == 2:
            if y is None:
                x, y = x
            else:
                raise ValueError('When passing a sequence at the x argument, the y argument must not be passed (received {0}).'.format(repr(y)))
        else:
            raise ValueError('The supplied sequence must have exactly 2 elements ({0} were received).'.format(len(x)))
    else:
        pass # x and y are just number values

    return x, y