Merging two associative arrays

[berni]

I've got two associative arrays and want to get a new one, which 
is created out of both of them:

This works:

string[int] a = [1:"one", 7:"seven"];
string[int] b = [5:"five", 9:"nine"];

string[int] tmp = a.dup;
foreach (k,v;b) tmp[k] = v;

assert(tmp==[1:"one", 7:"seven", 5:"five", 9:"nine"]);

But is there something easier, especially without making that 
"tmp" explicit. I hoped for a~b, but that didn't work. (I 
allready know, that there aren't duplicated keys, if that 
matters.)