message: Add __str__ method to _RefVariableProcessor.
diff: @@ -98,6 +98,9 @@ class _RefVariableProcessor(_OptimizableVariable):
  def __init__(self, v):
  self._v = v
  + def __str__(self):
  + return "<_RefVariableProcessor(%s)>" % self._v
  +
  def target(self):
  return self._v._ref() # pylint: disable=protected-access
  