var featureCtx = make(map[string]map[string]bool) // feature -> context name -> true
	for _, w := range walkers {
		for _, name := range w.stdPackages {
			pkg, err := w.import_(name)
			if _, nogo := err.(*build.NoGoError); nogo {
				continue
			}
			if err != nil {
				log.Fatalf("Import(%q): %v", name, err)
			}
			w.export(pkg)
		}

		ctxName := contextName(w.context)
		for _, f := range w.Features() {