The Epistle to the Romans uses μη γενοιτο (usually translated, "May it never be!" or "By no means!") five times. In four of those instances, it stands between a counterpoint and point that has αλλα as hinge (Ro 3.31; 7.7, 13; 11.11; the other instance is Ro 6.15). [On Counterpoints and Points, see this article on the Logos blog and also see this conference paper on negation by Dr. Steve Runge -- RB] Here's a sample, the <<..>> denote the CounterPoint (CP) and Point (P):

Ro 3.31:

<<CP νόμον οὖν καταργοῦμεν διὰ τῆς πίστεως;>> μὴ γένοιτο· ἀλλὰ <<νόμον ἱστάνομεν.>> (NA27)

<<CP Do we then overthrow the law by faith?>> By no means! On the contrary, <<P we uphold the law.>> (ESV)

So, Paul answers his own rhetorical question with the obvious answer (μη γενοιτο!), then uses αλλα to fill out the comparison/contrast.

In examining instances of αλλα in the Apostolic Fathers, I noticed one instance of the same thing going on in the Epistle of Barnabas:

Ep.Barn 6.3:

εἶτα τί λέγει; Καὶ ὃς ἐλπίσει ἐπʼ αὐτὸν ζήσεται εἰς τὸν αἰῶνα. <<CP ἐπὶ λίθον οὖν ἡμῶν ἡ ἐλπίς;>> μὴ γένοιτο· ἀλλʼ <<P ἐπεὶ ἐν ἰσχύϊ τέθεικεν τὴν σάρκα αὐτοῦ ὁ κύριος.>> λέγει γάρ· Καὶ ἔθηκέν με ὡς στερεὰν πέτραν. (Holmes 2nd Ed)

Then what does he say? “And whoever sets his hope on him will live forever.” <<CP Does our hope, then, rest on a rock?>> By no means! But <<P he says this because the Lord has established his flesh in strength.>> For he says: “And he established me like a solid rock.” (Holmes 2nd Ed)

The comparison/contrast is less straightforward in Ep.Barn., but you get the contrast, particularly when you look at the end of v. 2, " ... 'Behold, I will set into the foundations of Zion a precious stone, especially chosen, a cornerstone, highly valued.'" That rolls right into v. 3; all of it referring to Is 28.16. The author of Ep.Barn. is saying that this passage in Isaiah doesn't mean that a rock will save us, he points to a different passage of Isaiah (Is 50.7) to explain the rock reference; vv. 4-5 have three more citations doing the same thing. All in accordance with the allegorizing style of the letter. The bottom line is that it isn't a rock that saves us (that's ridiculous!), it is the Lord that saves us.

Anyway, I found it interesting that the Epistle of Barnabas uses, at least in this one instance, rhetoric similar to the Epistle to the Romans. No, I'm not saying that Paul wrote Barnabas (or that Barnabas, influenced by Paul, wrote Barnabas). I'm just noting a little gem I found while sifting through mounds of data.