When an upapada is compounded with an indeclinable, then it is compounded only with those avyayas which end in the affix am,
This makes a restriction, to the general compounding of upapadas with avyayas as ordained by the last rule. As स्वादुकारं भुङ्क्ते 'he eats having made his food sweet.' So also लवणकारं 'having seasoned. The avyayas ending in अम् are formed by the affix ण्मुल् 3.4.26 &c.
Why do we say 'with avyayas ending in अम्'? Observe कालो भोक्तुम् 'the time of eating.' Here the avyaya ends in उम् of the affix तुमुन् added by rule 3.3.167 (the affix tumun may be applied, when the word in construction is not a verb, but काल, समय or वेला 'time.')
The force of the word only is to indicate that this composition takes place in those cases where a rule ordains अम् only, after any root with regard to any upapada; so that no composition will take place where the affix अम् as well as another affix is ordained after a root with certain upapadas. Thus sutra 3.4.24, declares: \the affixes क्त्वा and णमुल् come after a verb when the words अमे, प्रथम and पूर्व are upapadas.\ Here the अम् (affix णमुल्) is not the sole affix ordained; but there is a co-ordinate affix with it namely क्त्वा. Therefore in अग्रेभोजम् 'having first eaten' there is no composition because अग्रेभोजम् is not the only form we can have; for, अग्रेभुक्त्वा is also used in the same sense.,