diff --git a/laydi/dataset.py b/laydi/dataset.py
index 1ab7f1b..00591eb 100644
--- a/laydi/dataset.py
+++ b/laydi/dataset.py
@@ -1,11 +1,80 @@
-from scipy import ndarray,atleast_2d,asarray,intersect1d,zeros,empty,sparse,\
-where
+from scipy import ndarray, atleast_2d, asarray, intersect1d, zeros
+from scipy import empty, sparse, where
 from scipy import sort as array_sort
 from itertools import izip
 import shelve
 import copy
 import re
 
+class Universe(object):
+    def __init__(self, name):
+        self.name = name
+        self._ids = {}
+
+    def register(self, dim):
+        """Increase reference count for identifiers in Dimension object dim"""
+        if dim.name != self.name:
+            return
+        for i in dim:
+            self._ids[i] = self._ids.get(i, 0) + 1
+    
+    def unregister(self, dim):
+        """Update reference count for identifiers in Dimension object dim
+           Update reference count for identifiers in Dimension object dim, and remove all
+           identifiers with a reference count of 0, as they do not (by definition) exist 
+           any longer.
+        """
+        if dim.name != self.name:
+            return
+        for i in dim:
+            refcount = self._ids[i]
+            if refcount == 1:
+                self._ids.pop(i)
+            else:
+                self._ids[i] -= 1
+
+    def __str__(self):
+        return "%s: %i elements, %i references" % (self.name, len(self._ids), sum(self._ids.values()))
+
+    def __contains__(self, element):
+        return self._ids.__contains__(element)
+
+    def __len__(self):
+        return len(self._ids)
+
+    def intersection(self, dim):
+        return set(self._ids).intersection(dim.idset)
+
+
+class Dimension(object):
+    """A Dimension represents the set of identifiers an object has along an axis.
+    """
+    def __init__(self, name, ids=[]):
+        self.name = name
+        self.idset = set(ids)
+        self.idlist = list(ids)
+
+    def __getitem__(self, element):
+        return self.idlist[element]
+
+    def __getslice__(self, start, end):
+        return self.idlist[start:end]
+
+    def __contains__(self, element):
+        return self.idset.__contains__(element)
+        
+    def __str__(self):
+        return "%s: %s" % (self.name, str(self.idlist))
+
+    def __len__(self):
+        return len(self.idlist)
+
+    def __iter__(self):
+        return iter(self.idlist)
+
+    def intersection(self, dim):
+        return self.idset.intersection(dim.idset)
+
 
 class Dataset(object):
     """The Dataset base class.