code: public LongStreamEx prepend(LongStream other) {
          return new LongStreamEx(LongStream.concat(other, stream()), context.combine(other));
      }
docs: Creates a lazily concatenated stream whose elements are all the elements
  of the other stream followed by all the elements of this stream. The
  resulting stream is ordered if both of the input streams are ordered, and
  parallel if either of the input streams is parallel. When the resulting
  stream is closed, the close handlers for both input streams are invoked.
  
  @param other the other stream
  @return this stream prepended by the other stream
  @see LongStream#concat(LongStream, LongStream)