list.go
func (l List) From(ctx context.Context) Context {
	ctx = newContext(ctx, l)
	return &contextT{ctx: ctx}
}
func (l List) From(ctx context.Context) Context {
	ctx = newContext(ctx, l)
	return &contextT{ctx: ctx}
}
// From returns a new context with key/value pairs copied both from
// the list and the context.