{ "stage_singleton_field":
  { "vars": ["fieldname", "transition", "location"]
  , "expression":
    { "type": "assert_non_empty"
    , "msg":
      ["No artifact specified in field", {"type": "var", "name": "fieldname"}]
    , "$1":
      { "type": "disjoint_map_union"
      , "msg":
        [ "Expecting (essentially) a single artifact in field"
        , {"type": "var", "name": "fieldname"}
        ]
      , "$1":
        { "type": "foreach"
        , "var": "src"
        , "range":
          {"type": "FIELD", "name": {"type": "var", "name": "fieldname"}}
        , "body":
          { "type": "disjoint_map_union"
          , "$1":
            { "type": "foreach"
            , "var": "artifact"
            , "range":
              { "type": "values"
              , "$1":
                { "type": "DEP_ARTIFACTS"
                , "dep": {"type": "var", "name": "src"}
                , "transition":
                  { "type": "var"
                  , "name": "transition"
                  , "default": {"type": "empty_map"}
                  }
                }
              }
            , "body":
              { "type": "singleton_map"
              , "key": {"type": "var", "name": "location"}
              , "value": {"type": "var", "name": "artifact"}
              }
            }
          }
        }
      }
    }
  }
}