C.29:7.1 - Preserve a reservation operation

A display that retains on-hand quantity n but omits reserved quantity r merges (n,r)=(1,0) and (1,1), although only the first permits another reservation. C.29.1:5.1 constructs the available quantity F(n,r)=n-r, compares reservation before and after mapping, and establishes the shared permission condition. It returns a quantity that supports the reservation question while retaining separate totals when another question needs them.