// second type parameter defaults to Object
sclass WithTrail<A, B> extends Var<A> {
  B trail;
  
  *() {}
  *(A value) { super(value); }
  *(B *trail, A value) { super(value); }
  
  toString {
    if (trail == null) ret super.toString();
    ret super.toString() + "\n" +
      indentx("[Trail: " + trail + "]");
  }
}