The affix ṭac comes optionally after an avyayībhāva samāsa ending with a neuter word whose final syllable is aऩ्,
The word अन् is understood here also. The word 'Neuter' qualifies, the second i. e. last term of the compound. This allows option, where as the previous sutra had made it compulsory. Thus प्रतिचर्मम्, or प्रतिचर्म, उपचर्मम् or उपचर्म ॥,