(TR-END-DURATION 'tr) denotes a duration of a time range tr.
(Tp= (Tp+ (Time-Range.Start-Time ?Time-Range)
(Time-Range.Duration ?Time-Range))
(Time-Range.End-Time ?Time-Range))
(=> (Time-Range ?Time-Range)
(Tp= (Tp+ (Time-Range.Start-Time ?Time-Range)
(Time-Range.Duration ?Time-Range))
(Time-Range.End-Time ?Time-Range)))
(=> (Time-Range.Duration $X $Y) (Duration $Y))
(=> (Time-Range.Duration $X $Y) (Time-Range $X))