Description: Replace antecedent in antecedent. Proposition 21 of Frege1879 p. 40. (Contributed by RP, 24-Dec-2019) (Proof modification is discouraged.)
Ref | Expression | ||
---|---|---|---|
Assertion | frege21 |
Step | Hyp | Ref | Expression |
---|---|---|---|
1 | frege9 | ||
2 | frege19 | ||
3 | 1 2 | ax-mp |