Description: A variable is non-free in a proposition if and only if it is so in its negation. (Contributed by BJ, 6-May-2019)
Ref | Expression | ||
---|---|---|---|
Assertion | nfnbi |
Step | Hyp | Ref | Expression |
---|---|---|---|
1 | orcom | ||
2 | nf3 | ||
3 | nf3 | ||
4 | notnotb | ||
5 | 4 | albii | |
6 | 5 | orbi2i | |
7 | 3 6 | bitr4i | |
8 | 1 2 7 | 3bitr4i |