Synax for variadic template

[Alex via Digitalmars-d-learn]

Hi all!
Say, I have

struct A(T...)
{
        T arr;
}
struct B(T...)
{
        T[] arr;
}

void main()
{
        A!(int[], double[]) a;
        a.arr[0] ~= 5;
        a.arr[0] ~= 6;
        static assert(!__traits(compiles, a.arr[0] ~= 3.5));
        a.arr[1] ~= 19.8;
        
        assert(a.arr[0] == [5,6]);
        assert(a.arr[1] == [19.8]);
        assert(a.arr[1].length == 1);
        assert(a.arr[0].length == 2);
        
        B!(int, double) b;
        //b.arr[0] ~= 5;
}

While struct A behaves like expected, how to get by with B?
What I want is just to abstract the array property out of 
template arguments...

Thanks in advance :)