[F&O] IBM-FO-032: Inconsistent function signatures

(IBM-FO-032): Sections 9.7.3 (op:subtract-dates) and 9.7.4 
(op:subtract-times) have "?" occurrence indicators in their signatures and 
state that their arguments may be the empty sequence. But the other op: 
functions in this section (such as 9.7.13, op:add-dayTimeDuration-to-time) 
do not have "?" occurrence indicators and do not accept empty sequences. 
What is the reason for this inconsistency?

--Don Chamberlin