Before a sārvadhātuka kit or ṅit affix, the a of śna and as is elided.,
श्न is the characteristic of the roots of the Rudhadi class, while अस् is the root itself. Thus रुन्धः; रुन्धति, भिन्तः, भिन्दन्ति; So also of अस् :- स्तः, सन्ति ॥ But भिनत्ति and अस्ति before non-कित् and non-ङित् affixes. श्न + अस् ought to be श्नास्, instead of that we have श्नस् in the sutra. The पररूप single substitution is an irregularity.,