diff --git a/.gitignore b/.gitignore
index bac4bda0..aee1f22c 100644
--- a/.gitignore
+++ b/.gitignore
@@ -13,6 +13,7 @@
 .nrepl-port
 /.lein-*
 /.nrepl-port
+Cargo.lock
 /checkouts/
 /classes/
 /node_modules/
diff --git a/src/lib.rs b/src/lib.rs
index cdfbe1aa..cd781274 100644
--- a/src/lib.rs
+++ b/src/lib.rs
@@ -1,6 +1,23 @@
+// Copyright 2016 Mozilla
+//
+// Licensed under the Apache License, Version 2.0 (the "License"); you may not use
+// this file except in compliance with the License. You may obtain a copy of the
+// License at http://www.apache.org/licenses/LICENSE-2.0
+// Unless required by applicable law or agreed to in writing, software distributed
+// under the License is distributed on an "AS IS" BASIS, WITHOUT WARRANTIES OR
+// CONDITIONS OF ANY KIND, either express or implied. See the License for the
+// specific language governing permissions and limitations under the License.
+
+pub fn add_two(a: i32) -> i32 {
+    a + 2
+}
+
 #[cfg(test)]
 mod tests {
+    use super::*;
+
     #[test]
     fn it_works() {
+        assert_eq!(4, add_two(2));
     }
-}
+}
\ No newline at end of file
diff --git a/tests/external_test.rs b/tests/external_test.rs
new file mode 100644
index 00000000..ea8ff658
--- /dev/null
+++ b/tests/external_test.rs
@@ -0,0 +1,16 @@
+// Copyright 2016 Mozilla
+//
+// Licensed under the Apache License, Version 2.0 (the "License"); you may not use
+// this file except in compliance with the License. You may obtain a copy of the
+// License at http://www.apache.org/licenses/LICENSE-2.0
+// Unless required by applicable law or agreed to in writing, software distributed
+// under the License is distributed on an "AS IS" BASIS, WITHOUT WARRANTIES OR
+// CONDITIONS OF ANY KIND, either express or implied. See the License for the
+// specific language governing permissions and limitations under the License.
+
+extern crate datomish;
+
+#[test]
+fn external_test() {
+    assert_eq!(4, datomish::add_two(2));
+}
\ No newline at end of file