Therefore, they cannot share a robot (resource) in this
interval unless they can make use of the freeing/reallocation actions. For example, span S1,8
can free its assigned resource R (from level 1) and give it to span S2,6 at level 2. After using
R for its entire duration, span S2,6 can free it after level 6 so span S1,8 can use it to finish
its remaining duration