diff --git a/test/unit_tests/image_stitcher_test.py b/test/unit_tests/image_stitcher_test.py
index 1ec60d4..37c6fd5 100644
--- a/test/unit_tests/image_stitcher_test.py
+++ b/test/unit_tests/image_stitcher_test.py
@@ -168,6 +168,7 @@ def merge_group_vertically(group):
 
 
 class Stitcher:
+
     @staticmethod
     def groupby(pairs, coord_getter):
         pairs = sorted(pairs, key=coord_getter)
@@ -179,14 +180,27 @@ class Stitcher:
 
     def merge_along_axis(self, pairs, axis):
 
+        def group_pairs_by_c1(pairs):
+            return self.groupby(pairs, c1_getter)
+
+        def group_by_c2(pairs):
+            return self.groupby(pairs, c2_getter)
+
+        def group_pairs_within_groups_by_c2(groups):
+            return map(group_by_c2, groups)
+
+        def merge_groups_along_orthogonal_axis(groups):
+            return map(group_merger, groups)
+
         c1_getter = make_coord_getter(f"{self.other_axis(axis)}1")
         c2_getter = make_coord_getter(f"{self.other_axis(axis)}2")
         group_merger = make_group_merger(axis)
 
-        groups = self.groupby(pairs, c1_getter)
-        groups = chain.from_iterable(map(rpartial(self.groupby, c2_getter), groups))
-        groups = map(group_merger, groups)
-        pairs = chain.from_iterable(groups)
+        groups_of_pairs_with_same_c1 = group_pairs_by_c1(pairs)
+        groups_of_groups_of_pairs_with_same_c1_and_c2 = group_pairs_within_groups_by_c2(groups_of_pairs_with_same_c1)
+        groups_of_pairs_with_matching_c1_and_c2 = chain(*groups_of_groups_of_pairs_with_same_c1_and_c2)
+        groups_of_merged_pairs = merge_groups_along_orthogonal_axis(groups_of_pairs_with_matching_c1_and_c2)
+        pairs = chain(*groups_of_merged_pairs)
 
         return pairs