class Expression(object):
    ....

+    def __setattr__(self, name, value):
+        """
+        Protect expression objects from being modified, so they can be safely shared, in particular in L{Expression.reduce}.
+        """
+        raise AttributeError("Not allowed to set \"{}\"".format(name))
+
+    def set_value(self, name, value):
+        """
+        Set a variable to a value. Should only be used in constructors to preserve sharable property of the objects.
+
+        @param name: Name of the attribute to set.
+        @type  name: C{str}
+
+        @param value: New value of the attribute.
+        @type  value: anything
+        """
+        object.__setattr__(self, name, value)