diff --git a/pymbolic/cse.py b/pymbolic/cse.py
index 700479a06ee18990e2fb0ed70cd644b2a0c1e799..9329a696c12a97975318d10e1631ab11e8dbeee2 100644
--- a/pymbolic/cse.py
+++ b/pymbolic/cse.py
@@ -68,7 +68,10 @@ class CSEMapper(IdentityMapper):
     map_call = map_sum
 
     def map_common_subexpression(self, expr):
-        # don't duplicate CSEs
+        # Avoid creating CSE(CSE(...))
+        #
+        # NOTE: This is not equivalent to isinstance--it's more specific,
+        # and for a reason!
         if type(expr) is prim.CommonSubexpression:
             return prim.wrap_in_cse(self.rec(expr.child), expr.prefix)
         else: