Read only delegate

[Edwin van Leeuwen via Digitalmars-d-learn]

Is there a way to make sure a delegate only reads state, without 
changing it? I tried annotating the delegate as const, but that 
does not seem to work.

```D
void main()
{
    import std.stdio : writeln;
    auto r = [0,1,2,3];
    auto f = delegate() const // Compiles even though we are 
changing r
    {
        import std.array : popFront;
        r.popFront;
    };

    r.writeln; // [0,1,2,3]
    f();
    r.writeln; // [1,2,3]
}
```