> <span class="kw">list</span>(<span class="dt">max.depth =</span> <span class="kw">c</span>(2L, 6L),
                                              <span class="dt">min_child_weight =</span> <span class="kw">c</span>(1L, 10L),
                                              <span class="dt">subsample =</span> <span class="kw">c</span>(<span class="fl">0.5</span>, <span class="fl">0.8</span>)),
                                <span class="dt">init_grid_dt =</span> <span class="ot">NULL</span>, <span class="dt"