New Post: possible bug?

I think input == v2 is an intermediate assertion to infer v3 == sum(input).

the postconditions fromvar v1 := mapper(input); var v2,v3 := reduce(v1); are:
v1 is a sequence of KV which is generated based on the sequence input by fill the filed key with 0
v2 is a sequence of int which projects the filed value from v1
sum (v2) == v3


It is not surporised that dafny needs you to provide a hints that input == v2 in order to prove v3 == sum(input);