code: func Use(c context.Context, b *Bundle, e *Extra) context.Context {
  	b.EnsureLoaded(c)
  	return context.WithValue(c, &contextKey, &boundBundle{b, e})
  }
docs: // Use replaces the template bundle in the context. There can be only one bundle
  // installed at any time.
  //
  // It also takes an Extra to be passed to bundle's DefaultArgs(...) callback
  // when using Render(...) or MustRender(...) top-level functions.
  //
  // DefaultArgs(...) can use the context and the given Extra to extract
  // information about the environment.