Metamath Proof Explorer


Theorem dfssf

Description: Equivalence for subclass relation, using bound-variable hypotheses instead of distinct variable conditions. (Contributed by NM, 3-Jul-1994) (Revised by Andrew Salmon, 27-Aug-2011) Avoid ax-13 . (Revised by Gino Giotto, 19-May-2023)

Ref Expression
Hypotheses dfssf.1 _ x A
dfssf.2 _ x B
Assertion dfssf A B x x A x B

Proof

Step Hyp Ref Expression
1 dfssf.1 _ x A
2 dfssf.2 _ x B
3 df-ss A B z z A z B
4 1 nfcri x z A
5 2 nfcri x z B
6 4 5 nfim x z A z B
7 nfv z x A x B
8 eleq1w z = x z A x A
9 eleq1w z = x z B x B
10 8 9 imbi12d z = x z A z B x A x B
11 6 7 10 cbvalv1 z z A z B x x A x B
12 3 11 bitri A B x x A x B