This isn't quite what's going on. A better reading might be "which is a";
"Ǝx s.t. x∈ℕ" (there exists an x such that x is in the naturals) is just being shortened to "Ǝx∋ℕ" (there exists an x in the naturals), or there exists an x which is in the naturals.
It's not really that different from the normal usage.
If that’s it, why is it using ∋ rather than ∈? I would expect “Ǝx∈ℕ”.
Oh, doh! I'd missed that.
Yeah, that's...an unusual choice.
> Ǝx∋ℕ
"There is an x such that the set of natural numbers is a member of x"?
Yeah, my bad. My eyes autocorrected it.