Description: This is as close as we can get to proving extensionality for "the" "universal" class without ax-ext . (Contributed by BJ, 24-Apr-2024) (Proof modification is discouraged.)
Ref | Expression | ||
---|---|---|---|
Assertion | bj-elabtru | |- ( A e. { x | T. } <-> A e. { y | T. } ) |
Step | Hyp | Ref | Expression |
---|---|---|---|
1 | bj-denoteslem | |- ( E. z z = A <-> A e. { x | T. } ) |
|
2 | bj-denoteslem | |- ( E. z z = A <-> A e. { y | T. } ) |
|
3 | 1 2 | bitr3i | |- ( A e. { x | T. } <-> A e. { y | T. } ) |