इगुपधज्ञाप्रीकिरः कः
SV: इगुपध-ज्ञा-प्री-किरः कः
AV: कृत्
Language Meaning: After a verb ending in a consonant but preceded by इ , उ or ऋ -long or short, and after the verbs ज्ञा ‘to know’, श्री ‘to please’ and कॄ ‘to scatter’, comes the affix क (अ)।
Literal Meaning:
inward verb(fish/live) rope carry open net upto object(reed) hand-held inward water exist hard hold exist
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इगु - verb(inward) object(fish/live) has become इगु. Because इ is crystalized as सुप् and तिङ्.
उ meant outward and was adopted as क्रिया विभक्ति (from डुकृञ् / भू) meant all verbs were outward facing.
इ meant inward and was adopted as objectifying विभक्ति (from इष?) - all objectifications are inward centric.
It is possible that because action was outward facing since time of LUCA, it evolved as उ and because objectification was inward centric it emerged as इ. and by inference we can say अ existed before emergence of उ इ.
One this is certain तिङ् emerged from उ and सुप् from इ. and all धातु are अ - that has both inward or outward contained within.