Return the weight corresponding to single power.

def _compute_value(power, wg):
    """Return the weight corresponding to single power."""
    if power not in wg:
        p1, p2 = power
        # y power
        if p1 == 0:
            yy = wg[(0, -1)]
            wg[power] = numpy.power(yy, p2 / 2).sum() / len(yy)
        # x power
        else:
            xx = wg[(-1, 0)]
            wg[power] = numpy.power(xx, p1 / 2).sum() / len(xx)
    return wg[power]