Axiomatic Form
Invariant: A truck is at only one location
at(truck,loc1,i) & loc1 ¹ loc2 É Ø at(truck,loc2,i)
Optimality: Do not return a package to a location
at(pkg,loc,i) & Ø at(pkg,loc,i+1) & i<j É Ø at(pkg,loc,j)
Simplifying: Once a truck is loaded, it should immediately move
Ø in(pkg,truck,i) & in(pkg,truck,i+1) & at(truck,loc,i+1) É Ø at(truck,loc,i+2)