Disable cse_global on create_lb_collision_rule for Sympy < 1.2

lbmpy/creationfunctions.py

@@ -349,7 +349,13 @@ def create_lb_collision_rule(lb_method=None, optimization={}, **kwargs):
         collision_rule = cse_in_opposing_directions(collision_rule)
     if cse_global:
         from pystencils.simp import sympy_cse
-        collision_rule = sympy_cse(collision_rule)
+        sp_ver = sp.__version__.split('.')
+        if int(sp_ver[0]) > 1 or int(sp_ver[1]) > 1:
+            collision_rule = sympy_cse(collision_rule)
+        else:
+            import warnings
+            warnings.warn('SymPy versions below 1.2 may perform common subexpression elimination incorrectly on LB'
+                          'collision rules. Ignoring cse_global.')
 
     if params['output'] and params['kernel_type'] == 'stream_pull_collide':
         cqc = lb_method.conserved_quantity_computation