<dt>f1</dt>
<dd><p>the fitted values from a flexible estimation technique
regressing Y on X. A vector of the same length as <code>Y</code>; if sample-splitting
is desired, then the value of <code>f1</code> at each position should be the result
of predicting from a model trained without that observation.</p></dd>


<dt>f2</dt>
<dd><p>the fitted values from a flexible estimation technique
regressing either (a) <code>f1</code> or (b) Y on X withholding the columns in
<code>indx</code>. A vector of the same len