code: public <S,R> Converter<S> concreteConversion(Function<? super Higher<W, T>,? extends S> narrow){
          return  new Converter<S>(){
  
  
              @Override
              public <R> R to(Function<S, R> fn) {
                  return fn.apply(narrow.apply(single));
              }
          };
      }
docs: Convert this Active to a new type via the underlying concrete type
  e.g. Given an Active List
  <pre>
  {@code
  Active<list,Integer> active = Active.of(ListX.of(1,2,3), ListX.Instances.definitions());
  }
  </pre>
  
  We can convert it to a set via concreteConversion
  
  <pre>
  {@code
  SetX<Integer> set = active.concreteConversion(ListX.<Integer>kindCokleisli())
  .to(ListX::toSetX());
  }
  </pre>
  Most cyclops-react types provide kindCokleisli implementations that convert a Higher Kinded encoding of the type
  back to the concrete type
  
  @param narrow Narrowing function (Cokleisli) to a concrete type
  @param <S> Concrete type
  @param <R> Return type
  @return Converter that works on the concrete type