code: func (bb *Builder) Blob() *Blob {
  	json, err := mapJSON(bb.m)
  	if err != nil {
  		panic(err)
  	}
  	ss, err := parseSuperset(strings.NewReader(json))
  	if err != nil {
  		panic(err)
  	}
  	h := blob.NewHash()
  	h.Write([]byte(json))
  	return &Blob{
  		str: json,
  		ss:  ss,
  		br:  blob.RefFromHash(h),
  	}
  }
docs: // Blob builds the Blob. The builder continues to be usable after a call to Build.