Add Remove One List From Another as an elixir til

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.