original valarray vi:

[0,1,2,3,4,5,6,7,8,9,10,11,12,13,14,15,16,17,18,19,20,21]

vi[gslice(0,[4,2],[4,6])]

[0,6,4,10,8,14,12,18]

vi[gslice(2,[4,2],[4,6])]

[2,8,6,12,10,16,14,20]

vi[gslice(0,[4,2],[4,8])] *= vi[gslice(2,[4,2],[4,6])]

[0,1,2,3,24,5,48,7,80,9,120,11,168,13,224,15,16,17,360,19,20,21]