The value of a duration, without a specified time line. We also support an existence assumption: an entity has existence only during its duration value.
(=> (Duration-Value ?A ?B) (And (Exists (?Z ?T) (And (Present ?T) (Time-Span ?Z) (Begins-At ?T ?Z) (Exists (?C) (And (Duration-Value ?Z ?C) (Identity ?C ?B))) (Begins-At ?T (HAS-EXISTENCE ?A TRUE)) (Forall (?W ?T1) (=> (And (Present ?T1) (/= ?T ?T1) (Time-Span ?W) (Begins-At ?T1 ?W) (Or (Precedes ?W ?Z) (Follows ?W ?Z))) (Exists (?E) (And (Not (Has-Existence ?A True)) (Begins-At ?T1 ?E))))))) (Exists (?U) (And (Time-Unit ?U) (Exists (?F) (And (The-Time-Unit ?A ?F) (Identity ?F ?U)))))))