Description: A pair of elements of a class is a subset of the class. Theorem 7.5 of Quine p. 49. (Contributed by NM, 22-Mar-2006) (Proof shortened by Andrew Salmon, 29-Jun-2011)
Ref | Expression | ||
---|---|---|---|
Assertion | prssg | |
Step | Hyp | Ref | Expression |
---|---|---|---|
1 | snssg | |
|
2 | snssg | |
|
3 | 1 2 | bi2anan9 | |
4 | unss | |
|
5 | df-pr | |
|
6 | 5 | sseq1i | |
7 | 4 6 | bitr4i | |
8 | 3 7 | bitrdi | |