find?_not_eq_head_dropWhile — Mathlib

English

If there exists x in l with not p x, then l.find? (¬ p) x equals some head of dropWhile p l.

Русский

Если существует x в l such, что ¬ p x, тогда l.find? (¬ p) x = some (head of dropWhile p l).

LaTeX

$$$$ l.find? (\\lambda x. \\neg p x) = (l.dropWhile p).head? $$$$

Lean4

theorem find?_not_eq_head_dropWhile (h : ∃ x ∈ l, ¬p x) :
    l.find? (fun x ↦ !(p x)) = some ((l.dropWhile p).head (by simpa using h)) :=
  by
  convert l.find?_eq_head_dropWhile_not ?_
  · simp
  · simpa using h

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