is there a merge for associative arrays

[Daniel Davidson]

The code below causes a crash. What is the idiomatic way to merge 
associative arrays? If there is a simple version that allows the 
value at a key to be clobbered by the value of the right hand 
operand when there is a collision, that is a start.

import std.stdio;
void main() {
   double[string] a = [ "foo" : 22.2 ];
   double[string] b = [ "bar" : 22.2 ];
   writeln(a+b);
}


Is there a way to do something like this and have opApply be 
called for '+'?
If so, is it a bad idea?

import std.stdio;

double[string] opApply(string op)(const double[string][] inputs 
...)
   if(op == "+") {
   double[string] result;
   foreach( map ; inputs ) {
     foreach( key, value ; map ) {
       auto pval = key in result;
       if(pval) {
         *pval += value;
       } else {
         result[key] = value;
       }
     }
   }
   return result;
}

void main() {
   double[string] a = [ "foo" : 22.2 ];
   double[string] b = [ "bar" : 22.2 ];
   double[string] c = [ "bar" : 5 ];
   auto d = a+b+c;
}