code: func (b *Builtin) Ref() Ref {
  	parts := strings.Split(b.Name, ".")
  	ref := make(Ref, len(parts))
  	ref[0] = VarTerm(parts[0])
  	for i := 1; i < len(parts); i++ {
  		ref[i] = StringTerm(parts[i])
  	}
  	return ref
  }
docs: // Ref returns a Ref that refers to the built-in function.