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doc/outputs/overlays.md

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+# Overlays
+Writing overlays is a common occurence when using a NixOS system. Therefore,
+we want to keep the process as simple and straightforward as possible.
+
+Any _.nix_ files declared in this directory will be assumed to be a valid
+overlay, and will be automatically imported into all [hosts](../concepts/hosts.md), and
+exported via `overlays.<channel>/<pkgName>` _as well as_
+`packages.<system>.<pkgName>` (for valid systems), so all you have to do is
+write it.
+
+## Example
+overlays/kakoune.nix:
+```nix
+final: prev: {
+  kakoune = prev.kakoune.override {
+    configure.plugins = with final.kakounePlugins; [
+      (kak-fzf.override { fzf = final.skim; })
+      kak-auto-pairs
+      kak-buffers
+      kak-powerline
+      kak-vertical-selection
+    ];
+  };
+}
+```