pdl> p $x = pdl '[9 4 3 2 8]'
[9 4 3 2 8]
pdl> $to_sub = $x->rotate(-1)->sever # MUST sever, or next mutates $x
pdl> $to_sub(-1) .= 0
pdl> p $to_sub
[4 3 2 8 0]
pdl> p $x - $to_sub
[5 1 1 -6 8]