original valarray vi

[0,1,2,3,4,5,6,7,8,9]

vi[0,2,3,4,7,8]

[0,2,3,4,7,8]

vi[0,2,3,4,7,8] += vi[0,2,3,4,7,8]

[0,1,4,6,8,5,6,14,16,9]