fix a broken case in gaussian

kwant/rmt.py

@@ -106,12 +106,14 @@ def gaussian(n, sym='A', v=1.):
     """
     if sym not in sym_list:
         raise ValueError('Unknown symmetry type.')
-    if (c(sym) or t(sym) == -1 or p(sym) == -1) and n % 2:
-        raise ValueError('Matrix dimension should be even in chosen symmetry'
-                         'class.')
-    else:
-        tau_z = np.array((n // 2) * [1, -1])
-        idx_x = np.arange(n) + tau_z
+    if (c(sym) or t(sym) == -1 or p(sym) == -1):
+        if n % 2:
+            raise ValueError('Matrix dimension should be even in'
+                             ' chosen symmetry class.')
+        else:
+            tau_z = np.array((n // 2) * [1, -1])
+            idx_x = np.arange(n) + tau_z
+
     if sym == 'CII' and n % 4:
         raise ValueError('Matrix dimension should be a multiple of 4 in'
                          'symmetry class CII.')

kwant/tests/test_rmt.py

@@ -19,6 +19,8 @@ def test_gaussian():
     for sym in rmt.sym_list:
         if sym not in ('A', 'D', 'AI'):
             assert_raises(ValueError, rmt.gaussian, 5, sym)
+        else:
+            rmt.gaussian(5, sym)  # Check it is allowed.
         h = rmt.gaussian(n, sym)
         if rmt.t(sym):
             t_mat = np.array(rmt.h_t_matrix[sym])