diff --git a/basen/src/left_outer_join.hf b/basen/src/left_outer_join.hf
new file mode 100644
index 000000000..5c4e007b7
--- /dev/null
+++ b/basen/src/left_outer_join.hf
@@ -0,0 +1,21 @@
+lhs = mod[0]
+    -> tee();
+rhs = mod[1]
+    -> tee();
+
+lhs -> [0]joined;
+rhs -> [1]joined;
+
+joined = join::<'tick,'static>()
+    -> map(|(k, (lhs, rhs))| (k, (lhs, Some(rhs))))
+    -> combined;
+
+lhs -> [pos]missed;
+rhs -> map(|(k, _v)| k) -> [neg]missed;
+
+missed = anti_join::<'tick,'static>()
+    -> map(|(k, v)| (k, (v, None)))
+    -> combined;
+
+combined = union()
+    -> mod;