s may optionally be substituted for the visarga before a hard guttural or a labial in the chandas (Vedas); but neither before pra , nor before a doubled word.,
Thus अयः पात्रम् or अयस्पात्रम् ॥ This is an example of non-compounds. In compounds, the स change is compulsory by 8.3.46: because the option of the present sutra is asiddha there, and it finds its scope in cases other than compounds. If the maxim प्रकरणे प्रकरणमसिद्धं न योगे योगः be applied, then the two sutras 8.3.46 and 8.3.49 belong to the same प्रकरणं and one is not asiddha with regard to the other. Then we could give examples of compounds also under this sutra: but then such compounds will also be governed by 8.3.46, and so the स would be compulsory.
विश्वतस्पात्रम् or विश्वतः पात्रम्, here the word विश्वतः is an Indeclinable and hence the rule 8.3.46, does not apply to it. उरु णः कारः or उरु ण स्कारः ॥ Here नस् is substituted for अस्मद्, and then the न is changed to ण by 8.4.27. The word कारः is a घञ् formed word.
Why do we say
ot before प्र and a doubled word\? Observe अग्निः प्र विद्वान् (Av. V. 26. 1), पुरुषः पुरुषः परि ॥
In सूर्यरश्मिर्हरिकेशः पुरस्तात् (Rig. X. 139. 1), स नः पावकः (Rig. I. 12. 10), the स change has not taken place, as all rules are optional in the Vedas.,