diff -r 340514805e40 -r 0d58f0301923 lemon/lemon_writer.h
--- a/lemon/lemon_writer.h	Tue Jun 14 23:21:31 2005 +0000
+++ b/lemon/lemon_writer.h	Wed Jun 15 10:10:59 2005 +0000
@@ -55,13 +55,31 @@
       template <class _ItemIdWriter>
       struct Constraints {
 	void constraints() {
-	  const Item item;
 	  bool b = writer.isIdWriter();
 	  ignore_unused_variable_warning(b);
 	  writer.writeId(os, item);
 	}
 	_ItemIdWriter& writer;
 	std::ostream& os;
+	const Item& item;
+      };
+
+    };
+
+    template <typename Item>
+    class ItemWriter {
+    public:
+
+      void write(std::ostream&, const Item&) {}
+      
+      template <class _ItemWriter>
+      struct Constraints {
+	void constraints() {
+	  writer.write(os, item);
+	}
+	_ItemWriter& writer;
+	std::ostream& os;
+	const Item& item;
       };
 
     };
@@ -352,7 +370,8 @@
     template <typename Writer, typename Map>
     NodeSetWriter& writeNodeMap(std::string name, const Map& map, 
 			    const Writer& writer = Writer()) {
-      checkConcept<concept::WriteMap<Node, typename Map::Value>, Map>();
+      checkConcept<concept::ReadMap<Node, typename Map::Value>, Map>();
+      checkConcept<_writer_bits::ItemWriter<typename Map::Value>, Writer>();
       writers.push_back(
 	make_pair(name, new MapWriter<Node, Map, Writer>(map, writer)));
       return *this;
@@ -517,7 +536,8 @@
     template <typename Writer, typename Map>
     EdgeSetWriter& writeEdgeMap(std::string name, const Map& map, 
 			    const Writer& writer = Writer()) {
-      checkConcept<concept::WriteMap<Edge, typename Map::Value>, Map>();
+      checkConcept<concept::ReadMap<Edge, typename Map::Value>, Map>();
+      checkConcept<_writer_bits::ItemWriter<typename Map::Value>, Writer>();
       writers.push_back(
 	make_pair(name, new MapWriter<Edge, Map, Writer>(map, writer)));
       return *this;
@@ -698,7 +718,8 @@
     template <typename Writer, typename Map>
     UndirEdgeSetWriter& writeUndirEdgeMap(std::string name, const Map& map, 
 					  const Writer& writer = Writer()) {
-      checkConcept<concept::WriteMap<UndirEdge, typename Map::Value>, Map>();
+      checkConcept<concept::ReadMap<UndirEdge, typename Map::Value>, Map>();
+      checkConcept<_writer_bits::ItemWriter<typename Map::Value>, Writer>();
       writers.push_back(
 	make_pair(name, new MapWriter<UndirEdge, Map, Writer>(map, writer)));
       return *this;
@@ -709,10 +730,8 @@
     /// Add a new directed map writer command for the writer.
     template <typename Map>
     UndirEdgeSetWriter& writeEdgeMap(std::string name, const Map& map) {
-      checkConcept<concept::WriteMap<Edge, typename Map::Value>, Map>();
-      writeUndirEdgeMap("+" + name, composeMap(forwardMap(graph), map));
-      writeUndirEdgeMap("-" + name, composeMap(backwardMap(graph), map));
-      return *this;
+      return writeEdgeMap<typename Traits::
+	template Writer<typename Map::Value>, Map>(name, map);
     }
 
     /// \brief Add a new directed map writer command for the writer.
@@ -721,7 +740,8 @@
     template <typename Writer, typename Map>
     UndirEdgeSetWriter& writeEdgeMap(std::string name, const Map& map, 
 				     const Writer& writer = Writer()) {
-      checkConcept<concept::WriteMap<Edge, typename Map::Value>, Map>();
+      checkConcept<concept::ReadMap<Edge, typename Map::Value>, Map>();
+      checkConcept<_writer_bits::ItemWriter<typename Map::Value>, Writer>();
       writeUndirEdge("+" + name, composeMap(forwardMap(graph), map), writer);
       writeUndirEdge("-" + name, composeMap(backwardMap(graph), map), writer);
       return *this;
@@ -1160,6 +1180,7 @@
     AttributeWriter& writeAttribute(const std::string& name, 
 				    const Value& value,
 				    const Writer& writer = Writer()) {
+      checkConcept<_writer_bits::ItemWriter<Value>, Writer>();
       writers.push_back(make_pair(name, new ValueWriter<Value, Writer>
       			       (value, writer)));
       return *this;