ordered hashset

OrderedHashSet:
    map     // {T, {node{T}}
    dq      // {T}
    add(x):
        if has(x):
            // don't change position in queue
            // as it's a subtle behavior, instead let
            // the caller explicity call remove(x) and add(x)
            return
        map[x] = dq.enqueue(x)
    bool has(x):
        return map.hasKey(x)
    bool remove(x):
        if !has(x):
            return false
        dq.removeNode(map[x])
        map.remove(x)
        return true
    [] foreach():
        for x in dq:
            yield x

// deque interface
dq:
    node enqueue(x):
    bool removeNode(node):