import logic data.sigma
open sigma
inductive List (T : Type) : Type := nil {} : List T | cons   : T → List T → List T open List notation h :: t  := cons h t notation `[` l:(foldr `,` (h t, cons h t) nil) `]` := l
check ∃ (A : Type₁) (x y : A), x = y
check ∃ (x : num), x = 0
check Σ (x : num), x = 10
check Σ (A : Type₁), List A