diff --git a/src/mapper/substitutor.py b/src/mapper/substitutor.py
index 2aaffc8756faddfbf293f7b3af3ac523a2908550..ceed26718c37b15a8a54ddc2497a33649f2cdcd3 100644
--- a/src/mapper/substitutor.py
+++ b/src/mapper/substitutor.py
@@ -3,7 +3,7 @@ import pymbolic.mapper
 
 
 
-class SubstitutionMapper(pymbolic.mapper.IdentityMapper):
+class SubstitutionMapperBase(object):
     def __init__(self, variable_assignments):
         self.assignments = variable_assignments
 
@@ -28,11 +28,19 @@ class SubstitutionMapper(pymbolic.mapper.IdentityMapper):
 
 
   
-def substitute(expression, variable_assignments = {}):
+class SubstitutionMapper(SubstitutionMapperBase, 
+        pymbolic.mapper.IdentityMapper):
+    pass
+
+
+
+
+def substitute(expression, variable_assignments = {},
+        mapper_class=SubstitutionMapper):
     import pymbolic.primitives as primitives
 
     new_var_ass = {}
     for k, v in variable_assignments.iteritems():
         new_var_ass[primitives.make_variable(k)] = v
 
-    return SubstitutionMapper(new_var_ass)(expression)
+    return mapper_class(new_var_ass)(expression)