Metamath Proof Explorer


Theorem frege21

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 φ ψ χ φ θ θ ψ χ

Proof

Step Hyp Ref Expression
1 frege9 φ θ θ ψ φ ψ
2 frege19 φ θ θ ψ φ ψ φ ψ χ φ θ θ ψ χ
3 1 2 ax-mp φ ψ χ φ θ θ ψ χ