code: func (immutable *Immutable) copy() *Immutable {
  	var root *node
  	if immutable.root != nil {
  		root = immutable.root.copy()
  	}
  	cp := &Immutable{
  		root:   root,
  		number: immutable.number,
  		dummy:  *newNode(nil),
  	}
  	return cp
  }
docs: // copy returns a copy of this immutable tree with a copy
  // of the root and a new dummy helper for the insertion operation.