ka is substituted for ṣa or ḍha before sa,
Thus for ष of विष् we have विवेक्ष्यते, अवेक्ष्यत्, विविक्षति ॥ For ढ of लिड् (लिढ 8.2.31) we have लेक्ष्यति, अलेक्ष्यत्, लिलिक्षति ॥
For the स of the affix स्य &c, is substituted ष by 8.3.59.
Why do we say \before स\? Observe पिनष्टि, लेढि ॥,