I’m dealing with a combinatorial problem. Given N vectors of integers of varied lengths, I want to find all possible ways to select one integer from each vector. Here’s my code, which is unfortunately not type-stable:

```
julia> const data = [[1,2], [11,12], [101]];
julia> function all_combinations(a::Vector{Vector{Int}})
combinations = Vector{Int}[]
for c in Iterators.product(a...)
push!(combinations, collect(c))
end
combinations
end
julia> all_combinations(data)
4-element Vector{Vector{Int64}}:
[1, 11, 101]
[2, 11, 101]
[1, 12, 101]
[2, 12, 101]
```

Type instability is present according to the output of `@code_warntype all_combinations(data)`

. Is there a neat way to rewrite the code in a type-stable manner? (Neither N nor the length of each vector is known at compile time.)