sclass VarBasedArithmeticEngine {
  new MapSO vars;
  
  abstract record MultiOp(LS inVars, S outVar) implements Runnable {
    abstract O perform(L values);
    
    run {
      setVar(outVar, perform(map getVar(inVars));
    }
  }
  
  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); }
}