Request: Simple Associative Sets

[bmeck]

I find that I encounter situations where I simply need a hash set it is quite similair to an associative array but I end up always wanting something like

{
    [Node] visited;
}

and I simply want to know if it is in the set and remove it optionally which Associative Arrays already have. I was wondering the thoughts on if this would be useful? Right now I use

{
    bool[Node] visited;
}

and never ever even touch the bools since if it is in the set its all the info I need...