[PD] monophonic behavior...(again) how to get the lowest note?

[federico]

i got the h_stack working for incoming midi notes in order to achieve 
monophonic portamento (see the attached patch).

however this method has a bug:

you press C3 (osc plays C3) [stack->push(C3), out: C3]
you press F3 (osc slides to F3)  [stack->push(F3), out: F3]
you release C3 (osc slides to C3) [stack->pop(), out: C3] (wrong! F3 now 
it is the only note pressed, but osc plays C3)

remembering how VAZ works, i'm thinking that it is better to get highest 
(and lowest) note pressed, rather than using a stack.
however i can't find a container that would do the job.

can you help me?