Maybe this relates to duplicates in sparse matrices - how they are treated now:

Per default they are summed up

```
julia> A = sparse([2,2,2,2], [1,1,1,1], [2,1,3,4])
2×1 SparseMatrixCSC{Int64, Int64} with 1 stored entry:
⋅
10
```

or aggregated using the function chosen as `combine`

argument:

```
julia> A = sparse([2,2,2,2], [1,1,1,1], [2,1,3,4], 2, 1, max)
2×1 SparseMatrixCSC{Int64, Int64} with 1 stored entry:
⋅
4
```

if only first (or last) argument shall be used use `(a,b) -> a`

or `(a,b), -> b`

```
julia> A = sparse([2,2,2,2], [1,1,1,1], [2,1,3,4], 2, 1, (a,b) -> a)
2×1 SparseMatrixCSC{Int64, Int64} with 1 stored entry:
⋅
2
```