Made the suggested changes

theano/gof/fg.py

@@ -153,7 +153,6 @@ class FunctionGraph(utils.object2):
 
         self.inputs = list(inputs)
         self.outputs = outputs
-        self._removed_nodes = set()
 
         for f in features:
             self.attach_feature(f)
@@ -326,7 +325,6 @@ class FunctionGraph(utils.object2):
                     self.apply_nodes.remove(apply_node)
                     self.variables.difference_update(apply_node.outputs)
                     self.execute_callbacks('on_prune', apply_node, reason)
-                    self._removed_nodes.add(apply_node)
                     for i, input in enumerate(apply_node.inputs):
                         self.__remove_clients__(input, [(apply_node, i)],
                                                 reason=reason)
@@ -481,11 +479,6 @@ class FunctionGraph(utils.object2):
 
         for node in new_nodes:
             assert node not in self.apply_nodes
-            prevent_addition = node in self._removed_nodes
-                    
-            if prevent_addition : 
-                raise InconsistencyError
-                ("Trying to reintroduce an old nodes in the graph. This should not happen")
             self.__setup_node__(node)
             self.apply_nodes.add(node)
             for output in node.outputs:

theano/gof/toolbox.py

@@ -266,6 +266,7 @@ class ReplaceValidate(History, Validator):
     pickle_rm_attr = (["replace_validate", "replace_all_validate",
                        "replace_all_validate_remove"] +
                       History.pickle_rm_attr + Validator.pickle_rm_attr)
+    _nodes_removed = set()
 
     def on_attach(self, fgraph):
         for attr in ('replace_validate', 'replace_all_validate',
@@ -347,6 +348,7 @@ class ReplaceValidate(History, Validator):
 
         """
         chk = fgraph.replace_all_validate(replacements, reason)
+        self._nodes_removed.update(remove)
         for rm in remove:
             if rm in fgraph.apply_nodes or rm in fgraph.variables:
                 fgraph.revert(chk)
@@ -369,6 +371,9 @@ class ReplaceValidate(History, Validator):
             del d["history"]
         return d
 
+    def on_import(self, fgraph, node, reason):
+        if node in self._nodes_removed:
+            raise theano.gof.InconsistencyError("Trying to introduce a removed node")
 
 class NodeFinder(Bookkeeper):