[FOM] independence of extensionality

[Thomas Forster]

Who was it who first proved the axiom of extensionality from the other 
axioms of ZF?  I'm thinking in particular of tricks like: all sets are 
empty (in the new sense)  unless they are singletons and $x \in \{y\}$
(in the new sense) if $x \in y$ (in the old).   Was it Fraenkel and 
Mostowski? And can anyone supply a reference..?

      Thomas




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