diff --git a/loopy/translation_unit.py b/loopy/translation_unit.py
index 2c663aa35a752b3fe3b4e54b7b16d2fa0b167b38..f690d99566e7380cdd83995b62da6866e1abb5ea 100644
--- a/loopy/translation_unit.py
+++ b/loopy/translation_unit.py
@@ -59,7 +59,6 @@ def _is_a_reduction_op(expr):
     if isinstance(expr, ResolvedFunction):
         return _is_a_reduction_op(expr.function)
 
-    from loopy.library.reduction import ReductionOpFunction
     return isinstance(expr, ReductionOpFunction)
 
 
@@ -458,11 +457,16 @@ class CallablesIDCollector(CombineMapper):
 def _get_reachable_callable_ids_for_knl(knl, callables):
     clbl_id_collector = CallablesIDCollector()
 
-    return frozenset().union(*(
-        _get_reachable_callable_ids_for_knl(callables[clbl].subkernel, callables) |
-        frozenset([clbl]) if isinstance(callables[clbl], CallableKernel) else
-        frozenset([clbl])
-        for clbl in clbl_id_collector.map_kernel(knl)))
+    def rec(clbl_id):
+        clbl = callables[clbl_id]
+        if isinstance(clbl, CallableKernel):
+            return (_get_reachable_callable_ids_for_knl(clbl.subkernel, callables)
+                    | frozenset([clbl_id]))
+        else:
+            return frozenset([clbl_id])
+
+    return frozenset().union(*(rec(clbl_id)
+                               for clbl_id in clbl_id_collector.map_kernel(knl)))
 
 
 def _get_reachable_callable_ids(callables, entrypoints):
@@ -475,6 +479,7 @@ def _get_reachable_callable_ids(callables, entrypoints):
 
 # {{{ CallablesInferenceContext
 
+
 def get_all_subst_names(callables):
     """
     Returns a :class:`set` of all substitution rule names in the callable