code: func NewWithFreeList(degree int, f *FreeList) *BTree {
  	if degree <= 1 {
  		panic("bad degree")
  	}
  	return &BTree{
  		degree: degree,
  		cow:    &copyOnWriteContext{freelist: f},
  	}
  }
docs: // NewWithFreeList creates a new B-Tree that uses the given node free list.