two ways.

trivially, accepted_pn = collect->pn about 20 lines up.

less trivially, 
- we accept a value with pn = 100
- election/whatever, new leader comes back with pn = 200
- we accept pn = 200, share our previous value with 100 (assuming we fix the trivial case above)
- election/whatever, new leader comes back with pn = 300
- we accept pn = 300, share our prevoius value with 200 (should be 100)