8.3.59 AdeshapratyayoHa
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In the same word, when a dental 's' is preceded by 'ku' or 'iNN', the dental 's' must be transformed into the cerebral 'SH'. There are also certain other contexts that permit this transformation.
Hence, we can see that 'agni+su' is transformed to 'agniSHu'. However, 'latA+su' is just 'latAsu'.