jaras may be substituted for jarā , optionally before a case-affix beginning with a vowel.,
As जरया or जरसा, in जरसा or जरयादन्ताः शीर्यन्ते ॥ जरायै or जरसे (जरसे or जरायै त्वा परिदद्युः) ॥ But only जराभ्याम्, जराभिः before an affix beginning with a consonant. The जरस् substitution takes place in the Neuter plural, when the augment नुम् is added: as अतिजरांसि ब्राह्मणकुलानि ॥ In अतिजरसं ब्राह्मणकुलं पश्य the affix is not elided. The form is thus evolved: अतिजर + अम् ॥ Here three rules present themselves simultaneously; first, luk-elision of the affix अम् by 7.1.23, (2) then the अम् substitution by 7.1.24, (3) and thirdly, जरस् for जर by this rule. Of these लुक्-elision is superseded by अम् of 7.1.24 which is an exception to 7.1.23; and in its turn अम् is replaced by the जरस् of this sutra. जरस् being substituted for जर, we have अतिजरस्, now luk cannot again appear and cause elision, as it has already lost the opportunity: and we are left with अम् alone, and have अतिजरसं ॥ In the Nominative Singular and Instrumental Plural we have अतिजरं and अतिजरैः according to the opinion of Gonardiya. The reason being सन्निपातलक्षणो विधिरनिमित्तं तद्विघातस्य 'that which is taught in a rule the application of which is occasioned by the combination of two things, does not become the cause of the destruction of that combination'. Because अम् was added to अतिजर because it ended in अ 7.1.24, and similarly भिः was replaced by ऐस् 7.1.9. Now अ has caused the production of अम् and ऐस्, therefore, these latter affixes, though beginning with a vowel will not cause जरस् to be substituted for जर by this rule, for then the produced will cause the destruction of the producer. Others hold that the above maxim is anitya, and we have Nominative Singular अतिजरसं, and Instrumental Plural अतिजरसैः ॥
The form अतिजरांसि is thus evolved अतिजर + इ ॥ Here if the नुम् augment be added first, it will be a portion of the anga, and will not be an intervention to anything which is to be added or operated upon the anga. But this augment will be an intervention with regard to जरा which is but a portion of the word अतिजर ॥ So that an operation applicable to जरा will not take effect, because of this नुम् intervention. And though tadantaviddhi applies in these chapters (पदाङ्गाधिकारे तस्य च तदन्तस्य च), yet the maxim is that the substitutes only replace those which are specifically exhibited in a rule (निर्दिश्यमानस्य आदेशा भवन्ति), therefore जरस् would not replace जरा which forms only a portion of a full word अतिजर ॥ Even if the substitution does take place, the नुम् would be found after the स् of जरस् ॥ Therefore, the जरस् substitution should be made first, because this is a subsequent rule; and having done so, the नुम् should be added afterwards under 7.1.72.
The form अतिजरसं is thus evolved. We have अतिजर + अम् ॥ Here on the maxim एकदेशविकृतस्य अनन्यत्वात्, we substitute जरस् for जर also, (for जर and जरा are considered as one). Then appears 7.1.23 ordaining the luk of अम् and 7.1.24, teaching अम् ॥ The latter rule is preferred for the reasons given above.,