Definition 1.1 (Extensionality). If A and B are sets, then A = B iff every element of A is also an element of B, and vice versa.

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Typo location:

Line 27 in d4e99d0

If $A$ and $B$ are sets, then $A = B$ iff

What is the typo: iff

Proposed correction: if

Not a typo. Short for if and only if.

Misunderstood , thanks for the clearup