ssing cost (<code>NA</code>) values for a particular zone
should also have missing (<code>NA</code>)  values in the argument to <code>solution</code>.</p></dd>


<dt><code>x</code> has <code>data.frame</code> planning units</dt>
<dd><p>The argument to <code>solution</code> must
be a <code>data.frame</code> with each column corresponding to a different zone,
each row corresponding to a different planning unit, and cell values
corresponding to the solution value. This means that if a <code>data.frame</code>
o