If there is more than one constraint (for example, both a volume limit and a weight limit, where the volume and weight of each item are not related), we get the multiply-con
As you said vol and weight both are positive quantities, try to use that fact that weight always decreases:
knap[position][vol][t]
Now t=0 when wt is positive, t=1 when wt is negative.
t=0
wt
t=1