Grammarians sometimes test badal by considering the verb or other governor as if repeated with the substitute.
جَاءَنِي أَخُوكَ زَيْدٌ.
“Your brother, namely Zayd, came to me.”
Removing «أَخُوكَ» leaves a workable core: «جَاءَنِي زَيْدٌ». This test helps explain the book’s poetic exception, without automatically turning all explanatory followers into badal.