code: FieldRef getCurrentAppendable() {
      FieldRef local = currentAppendable;
      if (local == null) {
        local =
            currentAppendable =
                FieldRef.createField(
                    owner, CURRENT_APPENDABLE_FIELD, LoggingAdvisingAppendable.class);
      }
      return local;
    }
docs: Returns the field that holds the currently rendering LoggingAdvisingAppendable object that is
  used for streaming renders.
  
  <p>Unlike normal variables the VariableSet doesn't maintain responsibility for saving and
  restoring the current renderee to a local.
  
  <p>TODO(lukes): it would be better if the VariableSet would save/restore... the issue is
  allowing multiple uses within a template to share the field.