+ # Fixing via ConfigTree.fix
+ s.t.fix('D')
+ s.t.fix(['E','A'])
+ s.parse('D=""; E=""; A=""; ')
+ s.assertRaises(cf.VariableFixedError, s.val, "D")
+ s.assertRaises(cf.VariableFixedError, s.val, "E")
+ s.assertRaises(cf.VariableFixedError, s.val, "A")