Theorem equcomi

Description: Commutative law for equality. Equality is a symmetric relation. Lemma 3 of KalishMontague p. 85. See also Lemma 7 of Tarski p. 69. (Contributed by NM, 10-Jan-1993) (Revised by NM, 9-Apr-2017)

	Ref	Expression
Assertion	equcomi	$$⊢x=y\to y=x$$

Proof

Step	Hyp	Ref	Expression
1		equid	$$⊢x=x$$
2		ax7	$$⊢x=y\to \left(x=x\to y=x\right)$$
3	1 2	mpi	$$⊢x=y\to y=x$$