{ "foo_composer":
  { "vars": ["FOO"]
  , "expression":
    { "type": "map_union"
    , "$1":
      [ {"type": "singleton_map", "key": "type", "value": "RESULT"}
      , { "type": "singleton_map"
        , "key": "artifacts"
        , "value":
          { "type": "singleton_map"
          , "key": "foo"
          , "value": {"type": "var", "name": "FOO"}
          }
        }
      ]
    }
  }
}