sa is substituted for saha , in a Name.,
Thus साश्वत्थम्, सपलाशम्, सशिंशपम् ॥ Why when it is a Name? Observe सहयुध्वा, सहकृत्वा ॥ The word स꣡ह has acute on the first syllable, because all Particles have acute on the first syllable. The substitute स coming in the room of the acute स and the grave ह, will have an accent mid-way between स꣡ acute and ह॒॒ grave, namely, it would have the svarita accent. But, as a matter of fact, it has the acute accent. As स꣡पुत्रः, स꣡भार्यः ॥ These are Bahuvrihi. In Avyayibhava, the samasa-accent will prevail 6.1.223, as सेष्टि꣡, सपशुबन्ध꣡म् ॥,