Lazy range of hashes?

[Andrej Mitrovic]

I could use something like this:

void main()
{
    int[string] x = ["foo":1];
    int[string] y = ["bar":1];
    assert("bar" in lazyHash(x, y));
}

Essentially it would turn into lazy 'in' checks, meaning first opIn_r
would be called for 'x', and then for 'y'.

Otherwise it might be expensive to have to create a new hash that has
all the keys and values of other hashes. Has anyone ever implemented
this?