code: func register(f *Fmt) {
  	if _, ok := fmts[f.Name]; ok {
  		fmt.Fprintf(os.Stderr, "%v is already defined: %#v", f.Name, f)
  		os.Exit(1)
  	}
  	fmts[f.Name] = f
  	lang := f.Language
  	if lang == "" {
  		lang = "general"
  	}
  	if langToFmts[lang] == nil {
  		langToFmts[lang] = make(map[string]*Fmt)
  	}
  	langToFmts[lang][f.Name] = f
  }
docs: // register must be called in init().