<p>If an existing <code>FFTrees</code> object <code>object</code> or <code>tree.definitions</code> are provided as inputs,
no new FFTs are created.
When both arguments are provided, <code>tree.definitions</code> take priority over the FFTs in an existing <code>object</code>.
Specifically,</p>
<ul><li><p>If <code>tree.definitions</code> are provided, these are assigned to the FFTs of <code>x</code>.</p></li>
<li><p>If no <code>tree.definitions</code> are provided, but an existing <code>FFTrees</code> object 