sa is optionally substituted for saha when the compound is a bahuvrīhi samāsa,
That compound in which all members are secondary-उपसर्जन-is a सर्वोपसर्जन, and the Bahuvrihi is such a compound, because in it all the members are secondary, the Principal being understood. Thus सपुत्रः or सहपुत्रः, सच्छात्रः or सहच्छात्रः ॥ Why do we say of a Bahuvrihi? Observe सहयुध्वा, सहकृत्वा ॥ The substitution does not however take place in a Bahuvrihi like सहकृत्वप्रियः or सहयुद्धप्रियः ॥,