137. In external combination

In external combination:

a. The final a or ā of a preposition, with initial ṛ of a root, makes ār instead of ar: Thus, ārchati (ā + ṛchati), avārchati (ava + ṛchati), upārṣati (ÇB.: upa + ṛṣati; but A.V. uparṣanti).

b. Instances are occasionally met with a final a or ā being lost entirely before initial e or o: thus, in verb-forms, av’ eṣyāmas AB., up’ eṣatu etc. AV.; in derivatives, as upetavya, upetṛ; in compounds, as daçoni, yathetam, and (permissibly) compounds with oṣṭha (not rare), otu (not quotable), odana, as adharoṣṭha or adharāuṣṭha, tilodana or tilāudana; and even in sentence-combination, as iv’ etayas, açvin’ eva, yath’ ociṣe (all RV.), tv’ odman B.; and always with the exclamation om or oṁkára.

c. The form ūh from √vah sometimes makes the heavier or vṛddhi (235) diphthongal combination with a preceding a-vowel; thus, prāuḍhi, akṣāuhiṇī (from pra + ūḍhi, etc.).