 based on the data that has been
 added to the model when this method is activated.
 <p>
 <code> predict(x) = intercept + slope * x </code></p>
 <p>
 <strong>Preconditions</strong>: <ul>
 <li>At least two observations (with at least two different x values)
 must have been added before invoking this method. If this method is
 invoked before a model can be estimated, <code>Double,NaN</code> is
 returned.
 </li></ul></p></div>
<dl><dt><span class="strong">Parameters:</span></dt><dd><code>x</code> - input <code