Add warning for taking commutator with SymPy objects

All SymPy objects should be commutative in drudge, so taking commutator
with SymPy objects is highly likely to be caused by an error.

drudge/drudge.py

@@ -13,7 +13,7 @@
 
 from IPython.display import Math, display
 from pyspark import RDD, SparkContext
-from sympy import IndexedBase, Symbol, Indexed, Wild, symbols, sympify
+from sympy import IndexedBase, Symbol, Indexed, Wild, symbols, sympify, Expr
 
 from .canonpy import Perm, Group
 from .drs import compile_drs, DrsEnv, DrsSymbol
@@ -786,6 +786,13 @@ def __ror__(self, other):
 
     def _comm(self, other, right=False):
         """Compute the commutator."""
+
+        if isinstance(other, Expr):
+            msg = 'Taking commutator with commutative expression `{}`'.format(
+                other
+            )
+            warnings.warn(msg)
+
         prod, free_vars, expanded = self._cartesian_terms(other, right)
 
         dumms = self._drudge.dumms