To the root kaś 'to go, to punish', is added the augment suṭ , when preceded by the preposition prati , the form being pratiṣkaśaḥ,
Verse
ग्राममद्य प्रवेक्ष्यामि भव मे त्वं प्रतिष्कशः ।
The word प्रतिष्कश is formed by adding अच् affix 3.1.134 to the root, with adding the prefix प्रति ॥ Thus ग्राममद्य प्रवेक्ष्यामि भव मे त्वं प्रतिष्कशः 'I shall inspect the town to-day, be thou my emissary'. The word प्रतिष्कशः means \a messenger, a herald, an emissary\. Why do we say \to the root कश\? Observe प्रतिगतः कशां = प्रतिकशोऽश्वः 'a horse guided by the whip'. Here though कशा is derived from कश, yet the augment does not take place, because the augment comes to the root कश्, and not to a derivative word.,