Fix mistake in CZ table generation

Using `combinations` instead of `combinations_with_replacement` was
leading to entries of the form [ ... , i, i] being excluded from the CZ
table.

abp/clifford.py

@@ -106,7 +106,7 @@ def get_cz_table(unitaries):
 
     # And now build the CZ table
     cz_table = np.zeros((2, 24, 24, 3))
-    rows = list(it.product([0, 1], it.combinations(range(24), 2)))
+    rows = list(it.product([0, 1], it.combinations_with_replacement(range(24), 2)))
                 # CZ is symmetric so we only need combinations
     for bond, (c1, c2) in tqdm(rows, desc="Building CZ table"):
         newbond, c1p, c2p = find_cz(bond, c1, c2, commuters, state_table)
@@ -135,7 +135,7 @@ except IOError:
     conjugation_table = get_conjugation_table(unitaries)
     times_table = get_times_table(unitaries)
     cz_table = get_cz_table(unitaries)
-
+    
     # Write it all to disk
     np.save("unitaries.npy", unitaries)
     np.save("conjugation_table.npy", conjugation_table)

tests/test_clifford.py

@@ -65,3 +65,14 @@ def test_conjugation_table():
 def test_times_table():
     """ Check the times table """
     assert clifford.times_table[0][4] == 4
+
+def test_cz_table_is_symmetric():
+    """ Test the CZ table is symmetric """
+    for bond, (a, b) in it.product([0, 1], it.combinations(xrange(24), 2)):
+        _, a1, a2 = clifford.cz_table[bond, a, b] 
+        _, b1, b2 = clifford.cz_table[bond, b, a]
+        assert (a1,a2) == (b2, b1)
+
+def test_cz_table_makes_sense():
+    """ Test the CZ table is symmetric """
+    assert all(clifford.cz_table[0, 0, 0] == [1, 0, 0])