Decreasing in a Sweater armhole

.... how to deal with decreases in the arm hole. The curve is perfect, no jogs or stair steps. The trick is to decrease one of the cast of stitches in the last two stitches of the preceding row. So for instance, if you are supposed to cast off three stitches in the next knit row, You would purl two together on the last two stitches of the preceding row (decreasing one stitch), and then only cast off TWO in the next row. The change in the line of decreases is amazing, and so much easier to sew up that all of those stair steps.