accept all scalar values (not only literals) in expr_walk_map

    const Expr* expr,

    const std::unordered_map<std::string, ExprVisitor>& fns) {

  for (; expr; expr = expr_next(expr)) {

    if (!expr_is_value_literal(expr)) {

    if (!expr_is_value(expr)) {

      return error(ERROR, "expected a literal");

    }