diff --git a/libdevcore/CommonData.h b/libdevcore/CommonData.h
index 4297f606f..ab4bfe682 100644
--- a/libdevcore/CommonData.h
+++ b/libdevcore/CommonData.h
@@ -166,6 +166,12 @@ template <class T, class U> std::vector<T>& operator+=(std::vector<T>& _a, U con
 		_a.push_back(i);
 	return _a;
 }
+/// Concatenate the contents of a container onto a set
+template <class T, class U> std::set<T>& operator+=(std::set<T>& _a, U const& _b)
+{
+	_a.insert(_b.begin(), _b.end());
+	return _a;
+}
 /// Concatenate two vectors of elements.
 template <class T>
 inline std::vector<T> operator+(std::vector<T> const& _a, std::vector<T> const& _b)
diff --git a/libsolidity/ast/AST.cpp b/libsolidity/ast/AST.cpp
index f4ab64910..ebc8bd480 100644
--- a/libsolidity/ast/AST.cpp
+++ b/libsolidity/ast/AST.cpp
@@ -95,10 +95,7 @@ set<SourceUnit const*> SourceUnit::referencedSourceUnits(bool _recurse, set<Sour
 			_skipList.insert(sourceUnit);
 			sourceUnits.insert(sourceUnit);
 			if (_recurse)
-			{
-				set<SourceUnit const*> referencedSourceUnits = sourceUnit->referencedSourceUnits(true, _skipList);
-				sourceUnits.insert(referencedSourceUnits.begin(), referencedSourceUnits.end());
-			}
+				sourceUnits += sourceUnit->referencedSourceUnits(true, _skipList);
 		}
 	}
 	return sourceUnits;