Multiset only verifying with one element array - verification

Question

I'm having a problem with verifying if the count of an element in a multiset is a specific number. Below are two programs, one working (verifying correctly) and one not.
Verifies correctly:
method Main() {
var a: array<int> := new int[1];
a[0] := 2;
assert a[0] == 2;
var ms: multiset<int> := multiset(a[..]);
print ms;
assert ms[2] == 1;
}
Doesn't verify correctly:
method Main() {
var a: array<int> := new int[2];
a[0] := 2;
a[1] := 3;
assert a[0] == 2;
assert a[1] == 3;
var ms: multiset<int> := multiset(a[..]);
print ms;
assert ms[2] == 1;
}
Although, using a sequence directly seems to work just fine:
method Main() {
var s := [2, 3];
var ms: multiset<int> := multiset(s);
print ms;
assert ms[2] == 1;
}
I'm not sure why this is happening. For the first program print ms; prints out multiset{2} and in the second program print ms; prints out multiset{2, 3} which both seem fine, but Dafny raises an assertion violation for the second one at assert ms[2] == 1;. It seems that this happens when the arrays length is > 1.
Am I misunderstanding how multisets work, or is there something else going wrong?

Answer 1

Solution 1
The second program verifies if you add assert a[..] == [2, 3]; some time after setting a[0] and a[1]. For example, the following works:
method Main() {
var a: array<int> := new int[2];
a[0] := 2;
a[1] := 3;
assert a[0] == 2;
assert a[1] == 3;
var ms: multiset<int> := multiset(a[..]);
assert a[..] == [2, 3];
print ms;
assert ms[2] == 1;
}
I don't know why that assert is necessary.
Solution 2
There's another solution which doesn't require asserting the concrete value of a[..]. One can define a function to_seq(a, i) which explicitly computes the value of a[i..] and then assert a[..] == to_seq(a, 0). In more detail:
function to_seq<T>(a: array<T>, i: int) : (res: seq<T>)
requires 0 <= i <= a.Length
ensures res == a[i..]
reads a
decreases a.Length-i
{
if i == a.Length
then []
else [a[i]] + to_seq(a, i + 1)
}
method Main() {
var a: array<int> := new int[2];
a[0] := 2;
a[1] := 3;
var ms: multiset<int> := multiset(a[..]);
assert a[..] == to_seq(a, 0);
assert ms[2] == 1;
}
I'm again not really sure why that assert helps.