But...

I'm rereading Steve Runge's Discourse Grammar of the Greek New Testament—again. Good stuff. I like this:

Although there are several contrastive or adversative particles, ἀλλά adds the unique constraint of correcting some aspect of what precedes. In terms of distinguishing ἀλλά from εἰ μή, the key is the relation of what follows the particle to what precedes. In the case of εἰ μή, the excepted element that replaces what precedes was a potential member of the negated set. In the case of ἀλλά, the correcting member was not a member of the original set; it is a new element. (page 93)

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That's all. Just wanted to share it : )
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