Theorem pm2.74

Description: Theorem *2.74 of WhiteheadRussell p. 108. (Contributed by NM, 3-Jan-2005) (Proof shortened by Andrew Salmon, 7-May-2011)

	Ref	Expression
Assertion	pm2.74	|- ( ( ps -> ph ) -> ( ( ( ph \/ ps ) \/ ch ) -> ( ph \/ ch ) ) )

Proof

Step	Hyp	Ref	Expression
1		orel2	|- ( -. ps -> ( ( ph \/ ps ) -> ph ) )
2		ax-1	|- ( ph -> ( ( ph \/ ps ) -> ph ) )
3	1 2	ja	|- ( ( ps -> ph ) -> ( ( ph \/ ps ) -> ph ) )
4	3	orim1d	|- ( ( ps -> ph ) -> ( ( ( ph \/ ps ) \/ ch ) -> ( ph \/ ch ) ) )