{ "for host":
  { "vars": ["ARCH", "HOST_ARCH"]
  , "expression":
    { "type": "singleton_map"
    , "key": "TARGET_ARCH"
    , "value":
      { "type": "var"
      , "name": "HOST_ARCH"
      , "default": {"type": "var", "name": "ARCH"}
      }
    }
  }
}