Fail checks for shift matrix

lbmpy/moments.py

@@ -454,7 +454,13 @@ def set_up_shift_matrix(moments, stencil, velocity_symbols=sp.symbols("u_:3")):
     M = moment_matrix(moments, stencil, shift_velocity=None)
     MN = moment_matrix(moments, stencil, shift_velocity=velocity_symbols)
 
-    return sp.simplify(MN * M.inv())
+    N = sp.simplify(MN * M.inv())
+
+    assert N.is_lower, "Calculating the shift matrix gave not a lower diagonal matrix. Thus it failed"
+    assert sum(N.diagonal()) == len(stencil), "Calculating the shift matrix failed. " \
+                                              "There are entries on the diagonal which are not equal to one"
+
+    return N
 
 
 def gram_schmidt(moments, stencil, weights=None):