uffix (default: <code>c("_x", "_y")</code>). Only fields documented as non static
in xm and ym are kept.</p>
    </div>
    <div class="section level2">
    <h2 id="details">Details<a class="anchor" aria-label="anchor" href="#details"></a></h2>
    <p>fun must have at least an x and a y parameters.
Each of them should be a data.frame with all the field values
given in xm and ym. Additional parameters
can be defined and will be forwarded using <code>...</code>.
fun should return a data frame with all the fie