M.L. West, Textual Criticism and Editorial Technique
(Stuttgart: B.G. Teubner, 1973; rpt. 2003), p. 58:
There is a dictum of Haupt, quoted with approval by Housman and others: "If the sense requires it, I am prepared to write Constantinopolitanus where the manuscripts have the monosyllabic interjection o". The point he is making is that emendation must start from the sense. But the failure to explain how Constantinopitanus came to be corrupted into o may leave others with certain doubts as to whether that is what the sense was. Until those doubts are stilled, the conjecture has the status of a diagnostic one.