code: public <S,R> Active<W,R> custom(Function<? super Higher<W, T>,? extends S> narrow,Function<? super S,? extends Higher<W,R>> fn){
          return Active.of(fn.apply(narrow.apply(single)),def1);
  
      }
docs: Perform a custom operation
  
  <pre>
  {@code
  Active<list,Integer> active = Active.of(ListX.of(1,2,3), ListX.Instances.definitions());
  Active<list, ListX<Integer>> grouped = active.custom(ListX::narrowK, l -> l.grouped(10));
  }
  </pre>
  
  @param narrow Function that narrows Higher Kinded encoding to it's concrete type
  @param fn Transformation function
  @param <S> Concrete type
  @param <R> Return type
  @return Transformed Active after custom operation