code: @NotNull
      public OptionalLong findFirst() {
          if (iterator.hasNext()) {
              return OptionalLong.of(iterator.nextLong());
          }
          return OptionalLong.empty();
      }
docs: Returns the first element wrapped by {@code OptionalLong} class.
  If stream is empty, returns {@code OptionalLong.empty()}.
  
  <p>This is a short-circuiting terminal operation.
  
  @return an {@code OptionalLong} with first element
  or {@code OptionalLong.empty()} if stream is empty