func dragRect(cvs *canvas.Canvas, m draw.Mouse, mc <-chan draw.Mouse) draw.Rectangle {
m0 := m
r := draw.Rectangle{m0.Point, m0.Point}
b := newBox(cvs, r)
cvs.Flush()
for m.Buttons != 0 {
m = <-mc
b.setRect(draw.Rectangle{m0.Point, m.Point}.Canon())
cvs.Flush()
}
b.delete(cvs)
cvs.Flush()
return draw.Rectangle{m0.Point, m.Point}.Canon()
}
