A Relationship between two Resources and an Activity or Activity-Spec whereby the one Resource may be used or consumed in the Activity or Activity-Spec instead of the other.
(Nth-Domain Resource-Substitute 3 Activity-Or-Spec)
(=> (Resource-Substitute ?Resource ?Resource-Sub ?A-Or-Spec) (=> (Can-Use-Resource ?A-Or-Spec ?Resource) (Can-Use-Resource ?A-Or-Spec ?Resource-Sub)))