From 56dd09058f7eaaca0813d9ffeb6980862b6bf303 Mon Sep 17 00:00:00 2001
From: Leonardo de Moura <leonardo@microsoft.com>
Date: Wed, 24 May 2017 12:56:02 -0700
Subject: [PATCH] feat(library/init/data/list/basic): add auxiliary list
 functions

---
 library/init/data/list/basic.lean | 13 +++++++++++++
 1 file changed, 13 insertions(+)

diff --git a/library/init/data/list/basic.lean b/library/init/data/list/basic.lean
index 36b44d167a..5f2442766b 100644
--- a/library/init/data/list/basic.lean
+++ b/library/init/data/list/basic.lean
@@ -65,6 +65,19 @@ protected def inter [decidable_eq α] : list α → list α → list α
 instance [decidable_eq α] : has_inter (list α) :=
 ⟨list.inter⟩
 
+protected def erase {α} [decidable_eq α] : list α → α → list α
+| []     b := []
+| (a::l) b := if a = b then l else a :: erase l b
+
+protected def bag_inter {α} [decidable_eq α] : list α → list α → list α
+| []      _   := []
+| _       []  := []
+| (a::l₁) l₂  := if a ∈ l₂ then a :: bag_inter l₁ (l₂.erase a) else bag_inter l₁ l₂
+
+protected def diff {α} [decidable_eq α] : list α → list α → list α
+| l      []      := l
+| l₁     (a::l₂) := if a ∈ l₁ then diff (l₁.erase a) l₂ else diff l₁ l₂
+
 def length : list α → nat
 | []       := 0
 | (a :: l) := length l + 1