coöccurrence

Gerhard Rohlfs, HGIS (3 vols., Bern: A. Francke, 1949–54), II, 282f. (= § 523), stressed the frequent coöccurrence of prefixation and choice of the -ire class endings in de-adjectival verbs (aggrandire, imbruttire, ribrunire — with which apparire, obviously, has little if anything in common) and tried to place the wavering in pres. ind. appaio ~ apparisco, etc. in a broader context (aborro ~ aborrisco, applaudo ~ applaudisco, etc.).