func (f *File) walk(x interface{}, context astContext, visit func(*File, interface{}, astContext)) {
	visit(f, x, context)
	switch n := x.(type) {
	case *ast.Expr:
		f.walk(*n, context, visit)

	// everything else just recurs
	default:
		f.walkUnexpected(x, context, visit)

	case nil:

	// These are ordered and grouped to match ../../go/ast/ast.go
	case *ast.Field:
		if len(n.Names) == 0 && context == ctxField {