code: public static Function<? super ReactiveSeq<Double>, ? extends ReactiveSeq<Double>> concatDoubles( ReactiveSeq<Double> b){
  
          return a->ReactiveSeq.fromSpliterator(DoubleStream.concat(a.mapToDouble(i->i),b.mapToDouble(i->i)).spliterator());
      }
docs: /*
  Fluent integer concat operation using primitive types
  e.g.
  <pre>
  {@code
  import static cyclops.ReactiveSeq.concatDoubles;
  
  ReactiveSeq.ofDoubles(1d,2d,3d)
  .to(concatDoubles(ReactiveSeq.ofDoubles(5,6,7,8,9)));
  
  //[1d,2d,3d,5d,6d,7d,8d,9d]
  }
  </pre>