code: public final <R> R as(IxFunction<? super Ix<T>, R> transformer) {
          return transformer.apply(this);
      }
docs: Calls the given transformers with this and returns its value allowing
  fluent conversions to non-Ix types.
  @param <R> the result type
  @param transformer the function receiving this Ix instance and returns a value
  @return the value returned by the transformer function
  @throws NullPointerException if transformer is null
  @since 1.0