Added existing_identifiers function to Dataset.

Added colouring drag'n'drop to DAGPlot in gobrowser module.

fluents/dataset.py

@@ -194,6 +194,12 @@ class Dataset:
                      for key in idents if self._map[dim].has_key(key)]
         return asarray(index)        
 
+    def existing_identifiers(self, dim, idents):
+        if not isinstance(idents, list) and not isinstance(idents, set):
+            raise ValueError("idents needs to be a list/set got: %s" %type(idents))
+
+        return [key for key in idents if self._map[dim].has_key(key)]
+
     def copy(self):
         """ Returns deepcopy of dataset.
         """