[tab] With no arguments, <lit>$vcv</lit> returns a square matrix
[tab] containing the estimated covariance matrix for the coefficients of
[tab] the last model. If the last model was a single equation, then you
[tab] may supply the names of two parameters in parentheses to retrieve
[tab] the estimated covariance between the parameters named
[tab] <argname>s1</argname> and <argname>s2</argname>.
[tab]