From Osher Doctorow
The axiom of comprehension says that for all z1, all z2,..., all zn,
there exists a y for all x such that "x is an element of y iff phi(x)
holds", where phi(x) is some predicate or formula involving x.
The axiom of extensionality says that for all x and for all y, "for
all z, 'u iff v'," iff x = y where u is (x is an element of x) and v
is (z is an element of y).
Osher Doctorow
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