For pāda is substituted pad when the former is bha,
The substitute replaces the whole form पाद् and not only the final, on the maxim निर्दिश्यमानस्यादेशा भवन्ति \substitutes take the place of that which is actually enunciated in a rule.\ ॥ Thus द्विपदः पश्य, द्विपदा, द्विपदे, द्विपदिकां ददाति 5.4.1, त्रिपदिकां ददाति, वैयाघ्रपद्यः ॥,