The Knapsack Problem
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Given a knapsack of a specified capacity and a number of objects, each with a weight and value,
pack the knapsack with objects such that we maximise the value of objects packed whilest
respecting the capacity constraint, i.e. do no overload the knapsack.

1. Edit the file knapsack.mzn
2. maximise the value of the knapsack
3. modify to give answer decision "Is there a packing of value greater than or equal to v"
   where v is a value taken on the command line
4. Investigate what happens as we vary v
5. How might we address the restriction "Take item i or item j, but not both"?