h2 id="details">Details<a class="anchor" aria-label="anchor" href="#details"></a></h2>
    <p><code>ft_timepoint()</code> - identifies the data field which should
be used as the independent time variable. There should be one and only one
of these specified.</p>
<p><code>ft_uniqueidentifier()</code> - identifies data fields which
contain a (usually computer-generated) identifier for an entity, e.g. a
patient. It does not need to be unique within the dataset.</p>
<p><code>ft_categorical()</code> - identifies 