code: func (b *Blob) StaticSetMergeSets() []blob.Ref {
  	if b.Type() != "static-set" {
  		return nil
  	}
  
  	s := make([]blob.Ref, 0, len(b.ss.MergeSets))
  	for _, ref := range b.ss.MergeSets {
  		if ref.Valid() {
  			s = append(s, ref)
  		}
  	}
  	return s
  }
docs: // StaticSetMergeSets returns the refs of the static-sets in "mergeSets". These
  // are the subsets of all the static-set members in the case of a large directory.