join of range of ranges?

[bearophile]

In some cases I'd like to join a range of ranges in a single 
array/string (I know here the inner map could be replaced by 
something better, this code is just an example):

import std.algorithm: map;
import std.array: join, array;
void main() {
    auto r1 = [1, 2]
              .map!(x => [1, 2].map!(y => '*').array)
              .join("_");
    auto r2 = [1, 2]
              .map!(x => [1, 2].map!(y => '*'))
              .join("_");
}


The code works only if I add an 'array' inside, to turn it into a 
range of arrays. Do you think it's right to ask as enhancement 
for std.array.join to work with a range of ranges too, as in the 
r2 case?

Bye,
bearophile