diff --git a/cozy/codegen/cxx.py b/cozy/codegen/cxx.py
index 9f3db8f5..5df0e0d8 100644
--- a/cozy/codegen/cxx.py
+++ b/cozy/codegen/cxx.py
@@ -10,7 +10,7 @@
     Type, INT, BOOL, TNative, TSet, TList, TBag, THandle, TEnum, TTuple, TRecord, TFloat,
     Exp, EVar, ENum, EFALSE, ETRUE, ZERO, ENull, EEq, ELt, ENot, ECond, EAll,
     EEnumEntry, ETuple, ETupleGet, EGetField,
-    Stm, SNoOp, SIf, SDecl, SSeq, seq, SForEach, SAssign)
+    Stm, SNoOp, SIf, SDecl, SSeq, seq, SForEach, SAssign, SCall)
 from cozy.target_syntax import TArray, TRef, EEnumToInt, EMapKeys, SReturn
 from cozy.syntax_tools import pprint, all_types, fresh_var, subst, free_vars, all_exps, break_seq, shallow_copy
 from cozy.typecheck import is_collection, is_scalar
@@ -272,6 +272,14 @@ def visit_EMakeMap2(self, e):
         self.declare(m, e)
         return m.id
 
+    def visit_EMapKeys(self, e):
+        key = self.fv(e.type.elem_type)
+        keys = self.fv(e.type)
+        self.declare(keys)
+        add_to_keys = SCall(keys, "add", [key])
+        self.visit(SForEach(key, e, add_to_keys))
+        return keys.id
+
     def visit_EHasKey(self, e):
         map = self.visit(e.map)
         key = self.visit(e.key)
diff --git a/examples/Makefile b/examples/Makefile
index b4452375..d1509441 100644
--- a/examples/Makefile
+++ b/examples/Makefile
@@ -23,7 +23,7 @@ select.h: select-flatmap.ds
 	cozy -t $(TIMEOUT) --allow-big-sets select-flatmap.ds --c++ select.h -p 8080 --verbose --save select.synthesized
 
 listcomp: listcomp.cpp listcomp.h
-	g++ -std=c++11 -O3 -Werror '$<' -o '$@'
+	g++ -std=c++11 -O3 -Wno-parentheses-equality -Werror '$<' -o '$@'
 
 run-listcomp: listcomp
 	time ./listcomp
diff --git a/examples/listcomp-flatmap.ds b/examples/listcomp-flatmap.ds
index 5b650a74..560ad750 100644
--- a/examples/listcomp-flatmap.ds
+++ b/examples/listcomp-flatmap.ds
@@ -22,3 +22,6 @@ ListComp02:
 
     op insert_s(s: S)
         Ss.add(s);
+
+    op delete_r(a: Int)
+        Rs.remove_all([ r | r <- Rs, r.A == a ]);