Description: Transitivity of subclass relationship. Exercise 5 of TakeutiZaring p. 17. (Contributed by NM, 24-Jun-1993) (Proof shortened by Andrew Salmon, 14-Jun-2011) Avoid axioms. (Revised by GG, 19-May-2025)
Ref | Expression | ||
---|---|---|---|
Assertion | sstr2 |
Step | Hyp | Ref | Expression |
---|---|---|---|
1 | imim1 | ||
2 | 1 | al2imi | |
3 | df-ss | ||
4 | df-ss | ||
5 | df-ss | ||
6 | 4 5 | imbi12i | |
7 | 2 3 6 | 3imtr4i |