Theorem ex-1st

Description: Example for df-1st . Example by David A. Wheeler. (Contributed by Mario Carneiro, 18-Jun-2015)

	Ref	Expression
Assertion	ex-1st	$$⊢1^{st}\left(⟨3,4⟩\right)=3$$

Proof

Step	Hyp	Ref	Expression
1		3ex	$$⊢3\in \mathrm{V}$$
2		4re	$$⊢4\in ℝ$$
3	2	elexi	$$⊢4\in \mathrm{V}$$
4	1 3	op1st	$$⊢1^{st}\left(⟨3,4⟩\right)=3$$