code: func NewReferenceFilteredIter(
  	ff func(r *plumbing.Reference) bool, iter ReferenceIter) ReferenceIter {
  	return &referenceFilteredIter{ff, iter}
  }
docs: // NewReferenceFilteredIter returns a reference iterator for the given reference
  // Iterator. This iterator will iterate only references that accomplish the
  // provided function.