The affix ṭhan and ṭhak comes after the word pratipatham , in the Accusative -2nd case in construction in the sense of 'who goes'.,
Thus प्रतिपथमेति = प्र꣡तिपथिकः or प्रातिपथिकः꣡ \who goes along the road\ the ठक् causes Vriddhi 7.2.118 whilst ठन् does not.,