sclass WithSideValue<A, B> extends Var<B> {
  A sideValue;
  
  *() {}
  *(A *sideValue, B value) { super(value); }
  *(B value) { super(value); }
  
  A sideValue() { ret sideValue; }
  
  toString { ret prependSquareBracketed(sideValue) + super.toString(); }
}