Or more generally: mit + superlative.
A friend of mine was reading something to me, when she suddenly said that.
The sentence was in the spirit of
Diese Variante ist mit am besten. (this variant is among the best)
After a short moment of alienation I remembered hearing that phrase before. I would like to know which grammatical functions mit takes on here.
According to Duden it's an adverb; if so, it's not a "normal" one (it cannot occupy the "Vorfeld" alone):
Es ist eingentlich am besten, dass [...]. / Eigentlich ist es am besten, dass [...].
Es ist mit am besten, dass [...]. / Mit ist es am besten, dass [...]. (sounds weird, even wrong)
So which kind of word is it precisely?