Added comment.

theano/gof/utils.py

@@ -116,6 +116,11 @@ def deprecated(filename, msg=''):
 
 
 def uniq(seq):
+    """
+    Do not use set, this must always return the same value at the same index.
+    If we just exchange other values, but keep the same pattern of duplication,
+    we must keep the same order.
+    """
     #TODO: consider building a set out of seq so that the if condition
     #is constant time -JB
     return [x for i, x in enumerate(seq) if seq.index(x) == i]