diff --git a/system/dataset.py b/system/dataset.py
index 5dfbe4e..75db010 100644
--- a/system/dataset.py
+++ b/system/dataset.py
@@ -91,7 +91,7 @@ class Dataset:
     def _set_identifiers(self,identifiers,all_dims):
         """Creates internal mapping of identifiers structure."""
         for dim,ids in identifiers:
-            pos_map={}
+            pos_map = ReverseDict()
             if dim not in self._dims:
                 self._dims.append(dim)
                 all_dims.add(dim)
@@ -139,10 +139,10 @@ class Dataset:
         else:
             return [dim for dim in self]
 
-    def get_identifiers(self, dim, indices=None,sorted=True):
+    def get_identifiers(self, dim, indices=None,sorted=False):
         """Returns identifiers along dim, sorted by position (index) is optional.
-
-        You can optionally provide a list of indices to get only the
+	
+        You can optionally provide a list/ndarray of indices to get only the
         identifiers of a given position.
 
         Identifiers are the unique names (strings) for a variable in a given dim.
@@ -154,21 +154,16 @@ class Dataset:
         except:
             pass
         
-        if sorted==True:
-            items = self._map[dim].items()
-            backitems = [ [v[1],v[0]] for v in items]
-            backitems.sort()
-            ids = [ backitems[i][1] for i in range(0,len(backitems))]
-            
-        else:
-            ids = self._map[dim].keys()
-
         if indices != None:
-            ids = [ids[index] for index in indices]
+            ids = [self._map[dim].reverse[i] for i in indices]
+	else:
+            if sorted==True:
+                ids = [self._map[dim].reverse[i] for i in array_sort(self._map[dim].values())]
+            else:
+                ids = self._map[dim].keys()
 
         return ids
         
-
     def get_indices(self, dim, idents=None):
         """Returns indices for identifiers along dimension.
 
@@ -182,6 +177,7 @@ class Dataset:
         else:
             index = [self._map[dim][key] for key in idents]
         return asarray(index)
+    
 
 class CategoryDataset(Dataset):
     """The category dataset class.
@@ -204,7 +200,7 @@ class CategoryDataset(Dataset):
     .
     """
     
-    def __init__(self,array,identifiers=None,name='A'):
+    def __init__(self,array,identifiers=None,name='C'):
         Dataset.__init__(self,array,identifiers=identifiers,name=name)
         self.has_dictlists = False
             
@@ -217,6 +213,7 @@ class CategoryDataset(Dataset):
         self.has_dictlists=True
         return data       
 
+
 class GraphDataset(Dataset):
     """The graph dataset class.
 
@@ -273,3 +270,19 @@ class Selection:
     def __init__(self):
         self.current_selection={}
         
+class ReverseDict(dict):
+    """
+    A dictionary which can lookup values by key, and keys by value.
+    All values and keys must be hashable, and unique.
+
+    d = ReverseDict((['a',1],['b',2]))
+    print d['a']
+    print d.reverse[1]
+    """
+    def __init__(self, *args, **kw):
+        dict.__init__(self, *args, **kw)
+        self.reverse = dict([[v,k] for k,v in self.items()])
+
+    def __setitem__(self, key, value):
+        dict.__setitem__(self, key, value)
+        self.reverse[value] = key