Hi there,

I’ve noticed something that isn’t well-documented imho.

When a sparse matrix has an empty column `i`

, what is `colptr[i]`

?

Example:

```
sparse(Matrix(S))
3×3 SparseMatrixCSC{Float64, Int64} with 3 stored entries:
0.360646 ⋅ ⋅
⋅ ⋅ 0.688464
0.230095 ⋅ ⋅
```

gives

```
S.colptr
4-element Vector{Int64}:
1
3
3
4
```

The Sparse Arrays documentation says

```
colptr::Vector{Ti} # Column j is in colptr[j]:(colptr[j+1]-1)
```

Can anyone elucidate?