a Relationship between an Activity or Activity-Spec and an Entity whereby the Entity is or can be used or consumed during the performance of the Activity or the Activities as specified in the Activity-Spec
(=> (Resource-Substitute ?Resource ?Resource-Sub ?A-Or-Spec) (=> (Can-Use-Resource ?A-Or-Spec ?Resource) (Can-Use-Resource ?A-Or-Spec ?Resource-Sub)))
(<=> (Resource ?X) (Exists (?Activity-Or-Spec) (Can-Use-Resource ?Activity-Or-Spec ?X)))