sclass VarBasedArithmeticEngine {
  MapSO vars = syncTreeMap();
  
  sclass NotRunYet {}
  static new NotRunYet notRunYet;
  
  abstract record MultiOp(LS inVars, S outVar) implements Runnable {
    O value = notRunYet;
    
    abstract O perform(L values);
    
    run { get(); }
    
    O get() {
      value = perform(map getVar(inVars);
      setVar(outVar, value);
      ret value;
    }
  }
  
  persistent class DoubleAdd > MultiOp {
    *(LS *inVars, S *outVar) {}
   
    Double perform(L values) {
      ret doubleSum(allToDouble(values));
    }
  }
  
  O getVar aka get(S var) { ret vars.get(var); }
  void setVar aka set(S var, O value) { vars.put(var, value); }
  
  *(O... _) {
    addParamsToMap_inPlace(vars, _);
  }
}