आदेश, लोप etc. acting on पर would be in place of आदि (not अल्).
For example, if तस्मादित्युत्तरस्य was to be applied for स्था and स्तम्भ् ( in उदः स्थास्तम्भोः पूर्वस्य i.e. of the previous / पूर्व after उद् ) then it would not be applied at आदि not अन्त्य.
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pending: define पर activity