English
Dedup of cons reduces to the if-case: dedup (a :: l) = if a ∈ l then dedup l else a :: dedup l.
Русский
Дедуп конc даёт выражение через ветвление: dedup (a :: l) = если a ∈ l то dedup l иначе a :: dedup l.
LaTeX
$$$$ dedup (a :: l) = if a \\in l then dedup l else a :: dedup l. $$$$
Lean4
theorem dedup_cons (a : α) (l : List α) : dedup (a :: l) = if a ∈ l then dedup l else a :: dedup l := by
simpa using dedup_cons' a l