code: void append2( long l, int x ) {
      if(_id == null || l != 0){
        if(_ls == null || _sparseLen == _ls.length) {
          append2slow();
          // again call append2 since calling append2slow might have changed things (eg might have switched to sparse and l could be 0)
          append2(l,x);
          return;
        }
        _ls[_sparseLen] = l;
        _xs[_sparseLen] = x;
        if(_id  != null)_id[_sparseLen] = _len;
        _sparseLen++;
      }
      _len++;
      assert _sparseLen <= _len;
    }
docs: Fast-path append long data