diff --git a/test/unit_tests/image_stitcher_test.py b/test/unit_tests/image_stitcher_test.py
index 86be406..f363d8d 100644
--- a/test/unit_tests/image_stitcher_test.py
+++ b/test/unit_tests/image_stitcher_test.py
@@ -49,15 +49,6 @@ x1_getter, y1_getter, x2_getter, y2_getter = map(make_coord_getter, ("x1", "y1",
 width_getter, height_getter = map(make_length_getter, ("width", "height"))
 
 
-def merge_group(group, axis="y"):
-
-    group = list(group)
-    current_pair = group.pop(0)
-    for pair in group:
-        if y2_getter(current_pair) == y1_getter(pair):
-            current_box = merge_pair_vertically(current_pair, pair)
-
-
 def merge_metadata_horizontally(m1, m2):
     m1, m2 = map(HorizontalKeyMapper, [m1, m2])
     return merge_metadata(m1, m2)
@@ -121,25 +112,73 @@ def concat_images(im1: Image, im2: Image, metadata: dict, axis):
     return im_aggr
 
 
+class Stitcher:
+    @staticmethod
+    def groupby(pairs, coord):
+        coord_getter = make_coord_getter(coord)
+        pairs = sorted(pairs, key=coord_getter)
+        return map(compose(list, second), groupby(pairs, coord_getter))
+
+    def stitch(self, pairs: Iterable[ImageMetadataPair]) -> ImageMetadataPair:
+        groups = self.groupby(pairs, "x1")
+        groups = chain.from_iterable(map(rpartial(self.groupby, "x2"), groups))
+        groups = map(partial(sorted, key=y1_getter), groups)
+        groups = map(merge_group, groups)
+
+
+def merge_group(group):
+    def f(pairs):
+        current_pair = pairs.pop(0)
+
+        to_remove = []
+
+        for pair in pairs:
+            if y2_getter(current_pair) == y1_getter(pair):
+                current_pair = merge_pair_vertically(current_pair, pair)
+                to_remove.append(pair)
+
+        return [current_pair, *filter(lambda p: p not in to_remove, pairs)]
+
+    pairs = list(group)
+
+    while True:
+        new_pairs = f(deepcopy(pairs))
+        if len(new_pairs) == len(pairs):
+            break
+        pairs = new_pairs
+
+    return new_pairs
+
+
 #####################################
 
 
+def test_merge_group(vertical_merge_test_pairs):
+    pr1, pr2, pr_merged_expected = vertical_merge_test_pairs
+    prs_merged = merge_group([pr1, pr2])
+    assert len(prs_merged) == 1
+    assert_pair_equal(prs_merged[0], pr_merged_expected)
+
+
+def assert_pair_equal(pr1, pr2):
+    assert pr1.metadata == pr2.metadata
+    assert images_equal(pr1.image, pr2.image)
+
+
 def test_merge_pairs_horizontally(horizontal_merge_test_pairs):
     pr1, pr2, pr_merged_expected = horizontal_merge_test_pairs
     pr_merged = merge_pair_horizontally(pr1, pr2)
-    assert pr_merged.metadata == pr_merged_expected.metadata
-    images_equal(pr_merged.image, pr_merged_expected.image)
+    assert_pair_equal(pr_merged, pr_merged_expected)
 
 
 def test_merge_pairs_vertically(vertical_merge_test_pairs):
     pr1, pr2, pr_merged_expected = vertical_merge_test_pairs
     pr_merged = merge_pair_vertically(pr1, pr2)
-    assert pr_merged.metadata == pr_merged_expected.metadata
-    images_equal(pr_merged.image, pr_merged_expected.image)
+    assert_pair_equal(pr_merged, pr_merged_expected)
 
 
 def images_equal(im1: Image, im2: Image):
-    assert np.allclose(image_to_normalized_tensor(im1), image_to_normalized_tensor(im2))
+    return np.allclose(image_to_normalized_tensor(im1), image_to_normalized_tensor(im2))
 
 
 @pytest.fixture
@@ -200,28 +239,15 @@ def test_concat_images_horizontally(horizontal_merge_test_metadata):
     im1, im2, im_merged_expected = map(random_size_gray_image_from_metadata, [mdat1, mdat2, mdat_merged])
     im_merged = concat_images_horizontally(im1, im2, mdat_merged)
     assert im_merged.size == im_merged_expected.size
-    images_equal(im_merged, im_merged_expected)
+    assert images_equal(im_merged, im_merged_expected)
 
 
 def test_concat_images_vertically(vertical_merge_test_metadata):
     mdat1, mdat2, mdat_merged = vertical_merge_test_metadata
-    im1, im2, im_merged_expected = map(random_single_color_image_from_metadata, [mdat1, mdat2, mdat_merged])
+    im1, im2, im_merged_expected = map(random_size_gray_image_from_metadata, [mdat1, mdat2, mdat_merged])
     im_merged = concat_images_vertically(im1, im2, mdat_merged)
     assert im_merged.size == im_merged_expected.size
-
-
-class Stitcher:
-    @staticmethod
-    def groupby(pairs, coord):
-        coord_getter = make_coord_getter(coord)
-        pairs = sorted(pairs, key=coord_getter)
-        return map(compose(list, second), groupby(pairs, coord_getter))
-
-    def stitch(self, pairs: Iterable[ImageMetadataPair]) -> ImageMetadataPair:
-        groups = self.groupby(pairs, "x1")
-        groups = chain.from_iterable(map(rpartial(self.groupby, "x2"), groups))
-        groups = map(partial(sorted, key=y1_getter), groups)
-        groups = map(merge_group, groups)
+    assert images_equal(im_merged, im_merged_expected)
 
 
 @pytest.mark.parametrize("width", [160])