On exit:
$p$, the number of parameters in the (sub)model, including the intercept if one is present. If
${\mathbf{what}}=\text{'S'}$, then the submodel is the one specified in
hform otherwise the model is the one used when defining the design matrix described in
hxdesc.
If ${\mathbf{lisx}}\ne 0$, if ${\mathbf{intcpt}}=\text{'Z'}$, $p={\sum}_{i=1}^{{m}_{x}}{\mathbf{isx}}\left(i\right)$, otherwise $p={\sum}_{i=1}^{{m}_{x}}{\mathbf{isx}}\left(i\right)+1$.