code: func (t *Treap) join(this *node, that *node) *node {
  	if this == nil {
  		return that
  	}
  	if that == nil {
  		return this
  	}
  	if this.priority > that.priority {
  		return &node{
  			item:     this.item,
  			priority: this.priority,
  			left:     this.left,
  			right:    t.join(this.right, that),
  		}
  	}
  	return &node{
  		item:     that.item,
  		priority: that.priority,
  		left:     t.join(this, that.left),
  		right:    that.right,
  	}
  }
docs: // All the items from this are < items from that.