{* *! version 1.0.0  16mar2007}{...}
    {cmd:vec(}{it:M}{cmd:)}
{p2colset 8 22 26 2}{...}
{p2col: Domain:}matrices{p_end}
{p2col: Range:}column vectors ({it:n} x 1 matrices){p_end}
{p2col: Description:}returns a column vector formed by listing the elements of
{it:M}, starting with the first column and proceeding column by column.{p_end}
{p2colreset}{...}
