Add Remove One List From Another as an elixir til

README.md

@@ -7,7 +7,7 @@ variety of languages and technologies. These are things that don't really
 warrant a full blog post. These are mostly things I learn by pairing with
 smart people at [Hashrocket](http://hashrocket.com/).
 
-_513 TILs and counting..._
+_514 TILs and counting..._
 
 ---
 
@@ -105,6 +105,7 @@ _513 TILs and counting..._
 - [Quitting IEx](elixir/quitting-iex.md)
 - [Range Into List Using Comprehensions](elixir/range-into-list-using-comprehensions.md)
 - [Referencing Values In IEx's History](elixir/referencing-values-in-iexs-history.md)
+- [Remove One List From Another](elixir/remove-one-list-from-another.md)
 - [Replace Duplicates In A Keyword List](elixir/replace-duplicates-in-a-keyword-list.md)
 - [Requiring Keys For Structs](elixir/requiring-keys-for-structs.md)
 - [Reversing A List](elixir/reversing-a-list.md)

elixir/remove-one-list-from-another.md

@@ -0,0 +1,29 @@
+# Remove One List From Another
+
+The `--/2` operator allows you to subtract two lists, that is, remove all
+elements in the _right_ list from the _left_ list. Each occurrence of an
+element is removed if there is a corresponding element. If there is no
+corresponding element, it is ignored.
+
+Here are some examples.
+
+```elixir
+> [1, 2, 3] -- [2, 4]
+[1, 3]
+> [:a, :b, :c, :a, :d, :a] -- [:a, :a]
+[:b, :c, :d, :a]
+```
+
+This kind of list operation is not particularly efficient, so for large
+lists it can be quite slow. The following example took several minutes to
+run.
+
+```elixir
+> Enum.into(1..1000000, []) -- Enum.into(2..1000000, [])
+[1]
+```
+
+To achieve a true set difference, you'll note that the docs for this
+operator recommend checking out `MapSet.difference/2`.
+
+See `h Kernel.--` for more details.