:</p><dl><dt><code>XS</code></dt>
<dd><p>Model matrix for the selection equation.</p></dd>

  <dt><code>YS</code></dt>
<dd><p>Response vector for the selection equation.</p></dd>

  <dt><code>NXS</code></dt>
<dd><p>Number of covariates in the selection model.</p></dd>

  <dt><code>XO</code></dt>
<dd><p>Model matrix for the outcome equation.</p></dd>

  <dt><code>YO</code></dt>
<dd><p>Response vector for the outcome equation.</p></dd>

  <dt><code>NXO</code></dt>
<dd><p>Number of covariates in the outcome mo