_add_node(queue, node) {
        // `score` is a measure of the merge priority: lower means higher priority
        // We use the BPE rank as a measure of priority (i.e., the local of the merge in the merges list)
        // We also add a fractional component to the score to break ties (with the earlier character having higher priority)
        const rank = this.bpe_ranks.get(JSON.stringify([node.token, node.next.token]));
        if (rank !== undefined) {
            node.score = rank + node.bias;
            queue.push(node);
        }
    }
Helper function to add a node to the priority queue.
@param {PriorityQueue} queue 
@param {BPENode} node
@private