replace

        fn replace(self: *Self, old: *Node, new: *Node) void {
            // copy over the values from the old node
            new.key = old.key;
            new.priority = old.priority;
            new.parent = old.parent;
            new.children = old.children;

            // point the parent at the new node
            const link = if (old.parent) |p| &p.children[@intFromBool(p.children[1] == old)] else &self.root;
            assert(link.* == old);
            link.* = new;

            // point the children's parent at the new node
            for (old.children) |child_node| {
                const child = child_node orelse continue;
                assert(child.parent == old);
                child.parent = new;
            }
        }