How to make a struct containing an associative array to deeply copy (for repeated usage in foreach) ?

[rkompass via Digitalmars-d-learn]

I want to make a custom dictionary that I may iterate through 
with foreach. Several times.
What I observe so far is that my dict as a simple forward range 
is exhausted after the first foreach and I have to deeply copy it 
beforehand.
With a simple associative array the exhaustion is not observed.
Is there a (hopefully simple) way to make this 
automatic/transparent? Of course
I need to use the struct.
Can I add a save member function? If yes: How? Or is there an 
operator that is used in the foreach initialization that I may 
overload in this struct?

My code:
```d
import std.stdio;
import std.string;
import std.typecons;

struct mydict {
   string[string] dct;

   @property bool empty() const {
      return dct.empty;
   }
   @property ref auto front() {
      return tuple(dct.keys[0], dct[dct.keys[0]]);
   }
        void popFront() {
                dct.remove(dct.keys[0]);
        }
        void opAssign(mydict rhs) {
                writeln("--opAssign--");
                foreach (k; rhs.dct.keys)  // do a deep copy
                        dct[k] = rhs.dct[k];
        }
}

void main() {

        mydict md, md2;
        md.dct = ["h":"no", "d":"ex", "r": "cow"];
        md2 = md;             // md2.opAssign(md)
        foreach (k, v; md)
                writeln("key: ", k, "val: ", v);
        writeln("----------");
	foreach (k, v; md)   // does not work with md again, md is 
exhausted
                writeln("key: ", k, "val: ", v);
}
```