Theorem mtpxor

Description: Modus tollendo ponens (original exclusive-or version), aka disjunctive syllogism, similar to mtpor , one of the five "indemonstrables" in Stoic logic. The rule says: "if ph is not true, and either ph or ps (exclusively) are true, then ps must be true". Today the name "modus tollendo ponens" often refers to a variant, the inclusive-or version as defined in mtpor . See rule 3 on Lopez-Astorga p. 12 (note that the "or" is the same as mptxor , that is, it is exclusive-or df-xor ), rule 3 of Sanford p. 39 (where it is not as clearly stated which kind of "or" is used but it appears to be in the same sense as mptxor ), and rule A5 in Hitchcock p. 5 (exclusive-or is expressly used). (Contributed by David A. Wheeler, 4-Jul-2016) (Proof shortened by Wolf Lammen, 11-Nov-2017) (Proof shortened by BJ, 19-Apr-2019)

	Ref	Expression
Hypotheses	mtpxor.min	$$⊢\neg \phi$$
	mtpxor.maj	$$⊢\left(\phi ⊻\psi \right)$$
Assertion	mtpxor	$$⊢\psi$$

Proof

Step	Hyp	Ref	Expression
1		mtpxor.min	$$⊢\neg \phi$$
2		mtpxor.maj	$$⊢\left(\phi ⊻\psi \right)$$
3		xoror	$$⊢\left(\phi ⊻\psi \right)\to \left(\phi \vee \psi \right)$$
4	2 3	ax-mp	$$⊢\left(\phi \vee \psi \right)$$
5	1 4	mtpor	$$⊢\psi$$