diff --git a/src/triacontagon/util.py b/src/triacontagon/util.py
index 2367b06..e6fefc8 100644
--- a/src/triacontagon/util.py
+++ b/src/triacontagon/util.py
@@ -63,7 +63,7 @@ def _clear_adjacency_matrix_except_rows(adjacency_matrix: torch.Tensor,
     rows = torch.cat(rows)
     print('cat took:', time.time() - t)
     # print('rows:', rows)
-    rows = set(rows.tolist())
+    # rows = set(rows.tolist())
     # print('rows:', rows)
 
     t = time.time()
@@ -71,7 +71,23 @@ def _clear_adjacency_matrix_except_rows(adjacency_matrix: torch.Tensor,
     indices = adj_mat.indices()
     values = adj_mat.values()
     print('indices[0]:', indices[0])
-    print('indices[0][1]:', indices[0][1], indices[0][1] in rows)
+    # print('indices[0][1]:', indices[0][1], indices[0][1] in rows)
+
+    lookup = torch.zeros(row_vertex_count * num_relation_types,
+        dtype=torch.uint8, device=adj_mat.device)
+    lookup[rows] = 1
+    values = values * lookup[indices[0]]
+    mask = torch.nonzero(values > 0, as_tuple=True)[0]
+    indices = indices[:, mask]
+    values = values[mask]
+
+    res = _sparse_coo_tensor(indices, values, adjacency_matrix.shape)
+    # res = res.coalesce()
+    print('res:', res)
+    print('"index_select()" took:', time.time() - t)
+
+    return res
+
     selection = torch.tensor([ (idx.item() in rows) for idx in indices[0] ])
     # print('selection:', selection)
     selection = torch.nonzero(selection, as_tuple=True)[0]
diff --git a/tests/triacontagon/test_util.py b/tests/triacontagon/test_util.py
index 5937535..e4f7f9d 100644
--- a/tests/triacontagon/test_util.py
+++ b/tests/triacontagon/test_util.py
@@ -78,6 +78,8 @@ def test_clear_adjacency_matrix_except_rows_03():
 def test_clear_adjacency_matrix_except_rows_04():
     adj_mat = (torch.rand(2000, 2000) < 0.001).to(torch.uint8)
 
+    print('adj_mat.to_sparse():', adj_mat.to_sparse())
+
     t = time.time()
     res = _sparse_diag_cat([ adj_mat.to_sparse() ] * 1300)
     print('_sparse_diag_cat() took:', time.time() - t)
@@ -93,3 +95,29 @@ def test_clear_adjacency_matrix_except_rows_04():
     truth = _sparse_diag_cat([ adj_mat.to_sparse() ] * 1300)
 
     assert _equal(res, truth).all()
+
+
+def test_clear_adjacency_matrix_except_rows_05():
+    if torch.cuda.device_count() == 0:
+        pytest.skip('Test requires CUDA')
+
+    device = torch.device('cuda:0')
+    adj_mat = (torch.rand(2000, 2000) < 0.001).to(torch.uint8).to(device)
+
+    print('adj_mat.to_sparse():', adj_mat.to_sparse())
+
+    t = time.time()
+    res = _sparse_diag_cat([ adj_mat.to_sparse() ] * 1300)
+    print('_sparse_diag_cat() took:', time.time() - t)
+
+    rows = torch.tensor(list(range(512)), device=device)
+
+    t = time.time()
+    res = _clear_adjacency_matrix_except_rows(res, rows,
+        2000, 1300)
+    print('_clear_adjacency_matrix_except_rows() took:', time.time() - t)
+
+    adj_mat[512:] = torch.zeros(2000)
+    truth = _sparse_diag_cat([ adj_mat.to_sparse() ] * 1300)
+
+    assert _equal(res, truth).all()