diff --git a/csst_proto/flip_image.py b/csst_proto/flip_image.py
index b8cb64389be7e2d242d7e7d5f7d1dd478636fd36..f49510f3d9e6512e2fa7f4bce6b82320b9c50f4a 100644
--- a/csst_proto/flip_image.py
+++ b/csst_proto/flip_image.py
@@ -4,7 +4,7 @@ from . import PACKAGE_PATH
 
 
 # the main algorithm
-def flip_image(img: np.ndarray):
+def flip_image(img: np.ndarray) -> np.ndarray:
     """ flip an input image
 
     This function uses an awesome algorithm to flip an input image.
@@ -16,14 +16,16 @@ def flip_image(img: np.ndarray):
 
     Returns
     -------
-    the flipped image
+    img_flipped: np.ndarray
+        the flipped image
 
     """
     try:
         assert img.ndim == 2
+        img_flipped = img[::-1, ::-1]
+        return img_flipped
     except AssertionError:
         raise AssertionError("The input image is {}D not 2D!".format(img.ndim))
-    return img[::-1, ::-1]
 
 
 # a demo on how to read test image
diff --git a/csst_proto/some_other_modules.py b/csst_proto/some_other_modules.py
index e69de29bb2d1d6434b8b29ae775ad8c2e48c5391..9be71bfa173e7fa6d43a908dc3b901a81c382993 100644
--- a/csst_proto/some_other_modules.py
+++ b/csst_proto/some_other_modules.py
@@ -0,0 +1,3 @@
+def some_arbitrary_functions():
+    """ an arbitrary function """
+    return
diff --git a/csst_proto/top_level_interface.py b/csst_proto/top_level_interface.py
index d5b7afbf84e2dad4c9d747c3779edee41b4328e8..8c9ad77b2883d489bef75c4a521a28f8dc6c6af4 100644
--- a/csst_proto/top_level_interface.py
+++ b/csst_proto/top_level_interface.py
@@ -1,4 +1,5 @@
 from .flip_image import flip_image, read_test_image
+from .some_other_modules import some_arbitrary_functions
 
 
-__all__ = ["flip_image", "read_test_image"]
+__all__ = ["flip_image", "read_test_image", "some_arbitrary_functions"]