Description: An extensionality-like principle defining subclass in terms of subsets. (Contributed by NM, 30-Jun-2004)
Ref | Expression | ||
---|---|---|---|
Assertion | ssextss | |
Step | Hyp | Ref | Expression |
---|---|---|---|
1 | sspwb | |
|
2 | dfss2 | |
|
3 | velpw | |
|
4 | velpw | |
|
5 | 3 4 | imbi12i | |
6 | 5 | albii | |
7 | 1 2 6 | 3bitri | |