feat(library/standard): add basic set theory that does not rely on classical axioms

Signed-off-by: Leonardo de Moura <leonardo@microsoft.com>

library/standard/set.lean

@@ -0,0 +1,64 @@
+--- Copyright (c) 2014 Jeremy Avigad. All rights reserved.
+--- Released under Apache 2.0 license as described in the file LICENSE.
+--- Author: Jeremy Avigad, Leonardo de Moura
+import logic funext bool
+using eq_proofs bool
+
+namespace set
+definition set (T : Type) := T → bool
+definition mem {T : Type} (x : T) (s : set T) := (s x) = '1
+infix `∈`:50 := mem
+
+section
+parameter {T : Type}
+
+definition empty : set T := λx, '0
+notation `∅`:max := empty
+
+theorem mem_empty (x : T) : ¬ (x ∈ ∅)
+:= assume H : x ∈ ∅, absurd H b0_ne_b1
+
+definition univ : set T := λx, '1
+
+theorem mem_univ (x : T) : x ∈ univ
+:= refl _
+
+definition inter (A B : set T) : set T := λx, A x && B x
+infixl `∩`:70 := inter
+
+theorem mem_inter (x : T) (A B : set T) : x ∈ A ∩ B ↔ (x ∈ A ∧ x ∈ B)
+:= iff_intro
+     (assume H, and_intro (band_eq_b1_elim_left H) (band_eq_b1_elim_right H))
+     (assume H,
+       have e1 : A x = '1, from and_elim_left H,
+       have e2 : B x = '1, from and_elim_right H,
+       calc A x && B x = '1 && B x : {e1}
+                   ... = '1 && '1  : {e2}
+                   ... = '1        : band_b1_left '1)
+
+theorem inter_comm (A B : set T) : A ∩ B = B ∩ A
+:= funext (λx, band_comm (A x) (B x))
+
+theorem inter_assoc (A B C : set T) : (A ∩ B) ∩ C = A ∩ (B ∩ C)
+:= funext (λx, band_assoc (A x) (B x) (C x))
+
+definition union (A B : set T) : set T := λx, A x || B x
+infixl `∪`:65 := union
+
+theorem mem_union (x : T) (A B : set T) : x ∈ A ∪ B ↔ (x ∈ A ∨ x ∈ B)
+:= iff_intro
+     (assume H, bor_to_or H)
+     (assume H, or_elim H
+       (assume Ha : A x = '1,
+        show A x || B x = '1, from Ha⁻¹ ▸ bor_b1_left (B x))
+       (assume Hb : B x = '1,
+        show A x || B x = '1, from Hb⁻¹ ▸ bor_b1_right (A x)))
+
+theorem union_comm (A B : set T) : A ∪ B = B ∪ A
+:= funext (λx, bor_comm (A x) (B x))
+
+theorem union_assoc (A B C : set T) : (A ∪ B) ∪ C = A ∪ (B ∪ C)
+:= funext (λx, bor_assoc (A x) (B x) (C x))
+
+end
+end