Theorem sylnbi

Description: A mixed syllogism inference from a biconditional and an implication. Useful for substituting an antecedent with a definition. (Contributed by Wolf Lammen, 16-Dec-2013)

	Ref	Expression
Hypotheses	sylnbi.1	|- ( ph <-> ps )
	sylnbi.2	|- ( -. ps -> ch )
Assertion	sylnbi	|- ( -. ph -> ch )

Proof

Step	Hyp	Ref	Expression
1		sylnbi.1	|- ( ph <-> ps )
2		sylnbi.2	|- ( -. ps -> ch )
3	1	notbii	|- ( -. ph <-> -. ps )
4	3 2	sylbi	|- ( -. ph -> ch )