diff --git a/set.hlean b/set.hlean
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+/-
+Copyright (c) 2017 Jeremy Avigad. All rights reserved.
+Released under Apache 2.0 license as described in the file LICENSE.
+Authors: Jeremy Avigad
+-/
+import types.trunc
+open funext eq trunc is_trunc
+
+definition set (X : Type) := X → Prop
+
+section
+open trunc_index
+
+definition propext {p q : Prop} (h : p ↔ q) : p = q :=
+tua (equiv_of_iff_of_is_prop h)
+
+end
+
+definition tempty {n : trunc_index} : (n.+1)-Type := trunctype.mk empty _
+
+namespace set
+
+variable {X : Type}
+
+/- membership and subset -/
+
+definition mem (x : X) (a : set X) := a x
+infix ∈ := mem
+notation a ∉ b := ¬ mem a b
+
+theorem ext {a b : set X} (H : ∀x, x ∈ a ↔ x ∈ b) : a = b :=
+eq_of_homotopy (take x, propext (H x))
+
+definition subset (a b : set X) : Prop := Prop.mk (∀⦃x⦄, x ∈ a → x ∈ b) _
+infix ⊆ := subset
+
+definition superset (s t : set X) : Prop := t ⊆ s
+infix ⊇ := superset
+
+theorem subset.refl (a : set X) : a ⊆ a := take x, assume H, H
+
+theorem subset.trans {a b c : set X} (subab : a ⊆ b) (subbc : b ⊆ c) : a ⊆ c :=
+take x, assume ax, subbc (subab ax)
+
+theorem subset.antisymm {a b : set X} (h₁ : a ⊆ b) (h₂ : b ⊆ a) : a = b :=
+ext (λ x, iff.intro (λ ina, h₁ ina) (λ inb, h₂ inb))
+
+-- an alterantive name
+theorem eq_of_subset_of_subset {a b : set X} (h₁ : a ⊆ b) (h₂ : b ⊆ a) : a = b :=
+subset.antisymm h₁ h₂
+
+theorem mem_of_subset_of_mem {s₁ s₂ : set X} {a : X} : s₁ ⊆ s₂ → a ∈ s₁ → a ∈ s₂ :=
+assume h₁ h₂, h₁ _ h₂
+
+/- empty set -/
+
+definition empty : set X := λx, tempty
+notation `∅` := empty
+
+theorem not_mem_empty (x : X) : ¬ (x ∈ ∅) :=
+assume H : x ∈ ∅, H
+
+theorem mem_empty_eq (x : X) : x ∈ ∅ = tempty := rfl
+
+/-
+theorem eq_empty_of_forall_not_mem {s : set X} (H : ∀ x, x ∉ s) : s = ∅ :=
+ext (take x, iff.intro
+  (assume xs, absurd xs (H x))
+  (assume xe, absurd xe (not_mem_empty _)))
+
+theorem ne_empty_of_mem {s : set X} {x : X} (H : x ∈ s) : s ≠ ∅ :=
+  begin intro Hs, rewrite Hs at H, apply not_mem_empty _ H end
+
+theorem empty_subset (s : set X) : ∅ ⊆ s :=
+take x, assume H, false.elim H
+
+theorem eq_empty_of_subset_empty {s : set X} (H : s ⊆ ∅) : s = ∅ :=
+subset.antisymm H (empty_subset s)
+
+theorem subset_empty_iff (s : set X) : s ⊆ ∅ ↔ s = ∅ :=
+iff.intro eq_empty_of_subset_empty (take xeq, by rewrite xeq; apply subset.refl ∅)
+
+lemma bounded_forall_empty_iff {P : X → Prop} :
+  (∀₀x∈∅, P x) ↔ true :=
+iff.intro (take H, true.intro) (take H, by contradiction)
+-/
+
+end set