sclass WithDuration<A> extends Var<A> {
  double seconds;
  
  *() {}
  *(double *seconds, A value) { super(value); }
  *(A value) { this(now(), value); }
  
  double seconds() { ret seconds; }
  
  toString { ret "[" + formatElapsedTimeWithAppropriateUnit(seconds) + "] " + super.toString(); }
}