doc(library/logic/axioms/examples): add alternative proof for has_left_inverse_of_injective

library/logic/axioms/examples/leftinv_of_inj2.lean

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+/-
+Copyright (c) 2015 Microsoft Corporation. All rights reserved.
+Released under Apache 2.0 license as described in the file LICENSE.
+
+Authors: Leonardo de Moura
+
+Classical proof that if f is injective, then f has a left inverse (if domain is not empty).
+This proof does not use Hilbert's choice, but the simpler axiom of choice which is just a proposition.
+-/
+import logic.axioms.classical
+open function
+
+-- The forall_not_of_not_exists at logic.axioms uses decidable.
+theorem forall_not_of_not_exists {A : Type} {p : A → Prop}
+  (H : ¬∃x, p x) : ∀x, ¬p x :=
+take x, or.elim (em (p x))
+  (assume Hp : p x,   absurd (exists.intro x Hp) H)
+  (assume Hnp : ¬p x, Hnp)
+
+theorem has_left_inverse_of_injective {A B : Type} {f : A → B} : nonempty A → injective f → ∃ g, ∀ x, g (f x) = x :=
+suppose nonempty A,
+assume inj : ∀ a₁ a₂, f a₁ = f a₂ → a₁ = a₂,
+have ∃ g : B → A, ∀ b, (∀ a, f a = b → g b = a) ∨ (∀ a, f a ≠ b), from
+  axiom_of_choice (λ b : B, or.elim (em (∃ a, f a = b))
+    (suppose (∃ a, f a = b),
+       obtain w `f w = b`, from this,
+       exists.intro w (or.inl (take a,
+         suppose f a = b,
+         have    f w = f a, begin subst this, exact `f w = f a` end,
+         inj w a `f w = f a`)))
+    (suppose ¬(∃ a, f a = b),
+     obtain a, from `nonempty A`,
+     exists.intro a (or.inr (forall_not_of_not_exists this)))),
+obtain g hg, from this,
+exists.intro g (take a,
+or.elim (hg (f a))
+  (suppose (∀ a₁, f a₁ = f a → g (f a) = a₁),
+    this a rfl)
+  (suppose (∀ a₁, f a₁ ≠ f a),
+    absurd rfl (this a)))