/* The database example */ // at time=1 Bill is hired and showed poor communication skills, and he is // thus assigned to administration source(2): [1]administration&-marketing&-communication_skill // at time=2 Bill is moved to marketing source(2): [2]-administration&marketing&communication_skill // department may change change(1) // skills don't change change(5): communication_skill change(5): -communication_skill // things discovered later may modify the initial knowledge backward // we can find where he work query: [1]administration query: [2]marketing // we can prove nothing about his skills query: [1]communication_skill query: [1]-communication_skill query: [2]communication_skill query: [2]-communication_skill