The affix ṭhak comes in the sense of 'mostly to be found there' after the words 1. upajānu 2. upakaraṇa and 3. uponīvi , being in the Locative -7th case.,
This debars अण् &c. Thus औपजानुकः, औपकर्णिकः औपनीविकः ।,
उपजानु-उपकर्ण-उपनीवेष् ठक्
Adhyāya 4 · Pāda 3 · Rule 40