ru is substituted for the m of pum (whereby the u is changed to uँ or uṃ) before a surd mute (khay) which is followed by a vowel, semi-vowel or a nasal (am pratyahāra).,
Thus पुँस्कामा or पुंस्कामा; पुँस्पुत्रः or पुंस्पुत्रः; पुँस्फलं or पुंस्फलम्; पुँश्चली or पुंश्चली ॥ The visarga in पुंस्कामा required to be changed optionally to ᳶ jihva-muliya by 8.3.37, but it is not so done. It is changed always to स् here by 8.3.34, also. The स् of पुंस् is dropped by 8.2.23, and the preceding sound is म, which comes to light in forms like पुमान् &c. For the sake of distinctness, Panini has elected to exhibit the shorter form पुम् when in the beginning of a composition : because this shorter form पुम् is the real stem in composition. खय् pratyahara includes the ten hard consonants, and अम् pratyahara includes all vowels, semivowels and nasals. पुंस्कामा is formed by the affix ण = पुमांसं कामयते ॥
In that alternative when we read the preceding sutra as समः स्सुटि, then the anuvritti of this स् will be current in the present sutra also, and so there will be no scope for the operation of 8.3.37. And though the anuvritti of रु is also current, yet it will not apply here, because of its non-appropriateness in that alternative.
Why do we say 'before a surd mute'? Observe पुंदासः, पुंगवः 5.4.92. Why do we say 'followed by a vowel or semivowel or nasal'? Observe पुंक्षीरः, पुंक्षुरः ॥ Why do we use the word परे in the sutra? Had we not used it, the sutra_ might have been open to this construction also. The म् of पुम् is so changed before a खय् letter which has an अम् letter, (whether preceding it or following it). So that the rule would have applied to पुमाख्यः, पुमाचारः ॥,