t.persp_{xlab, ylab, zlab}</code>: Labels of the x-, y- and z-
      axis. The defaults are <code>expression(x[1])</code>,
      <code>expression(x[2])</code> and <code>expression(f(x[1], x[2]))</code></p></li>
<li><p><code>bt.persp_ticktype</code>: Should the values of each dimension
      be shown in detail (<code>"detailed"</code>) or just via <code>"simple"</code>
      arrows in direction of increasement along the axes? The default is
      <code>"detailed"</code>.</p></li>
<li><p><code>bt.col_root</co