class_docstr: A proxy to access variables defined in a layer. 
code: def _add_variable(self, name, var):
      assert name not in self._vars
      self._vars[name] = var
  
imports: import six
label_desc: Use image node_id 2 for calling the VariableHolder obj's underlying member method code with example usage: obj._add_variable(name, var) without return types