code: func (immutable *Immutable) Insert(entries ...Entry) (*Immutable, Entries) {
  	if len(entries) == 0 {
  		return immutable, Entries{}
  	}
  
  	overwritten := make(Entries, 0, len(entries))
  	cp := immutable.copy()
  	for _, e := range entries {
  		overwritten = append(overwritten, cp.insert(e))
  	}
  
  	return cp, overwritten
  }
docs: // Insert will add the provided entries into the tree and return the new
  // state.  Also returned is a list of Entries that were overwritten.  If
  // nothing was overwritten for an Entry, a nil is returned in its place.