English
Let s ⊆ ι, t: ∀ i, Set(α i), and a(i) ∈ α i. Then (insert i s).sigma t equals Sigma.mk i '' t i ∪ s.sigma t.
Русский
Пусть s ⊆ ι, t: ∀ i, Set(α i), и a(i) ∈ α i. Тогда (insert i s).sigma t равно Sigma.mk i '' t i ∪ s.sigma t.
LaTeX
$$$(\mathrm{insert}\ i\ s) \sigma t = \Sigma i, t i \;''\; t i \cup s \sigma t$$$
Lean4
theorem insert_sigma : (insert i s).sigma t = Sigma.mk i '' t i ∪ s.sigma t := by grind