diff --git a/docs/langs/explicit-substitution/reduce/doAp.cic b/docs/langs/explicit-substitution/reduce/doAp.cic
new file mode 100644
index 00000000..9bf6426b
--- /dev/null
+++ b/docs/langs/explicit-substitution/reduce/doAp.cic
@@ -0,0 +1,12 @@
+import { Exp } from "../exp/index.cic"
+import { Mod } from "../mod/Mod.cic"
+import { reduce } from "./reduce.cic"
+
+export function doAp(mod: Mod, target: Exp, arg: Exp): Exp {
+  match (target) {
+    case Exp::Fn(name, ret) =>
+      reduce(mod, Exp::Let([new Binding(name, arg)], ret))
+
+    case _ => Exp::Ap(target, arg)
+  }
+}