test/model/default/classes/third.yml

classes:
- second

parameters:
  _param:
    notfound: exist
  myparam: ${_param:some}
  will:
    not:
      fail:
        at:
          tree: ${_param:notfound}
  three: ${two}
  empty:
    list: []
    dict: {}
  ~list_to_override: ${empty:list}
  ~dict_to_override: ${empty:dict}