Check that argument is a matrix -------------------------------
^matchk^ argname
Description -----------
^matchk^ checks that argname names an existing matrix. If not, an error message will be issued.
Remarks -------
^matchk^ is likely to be most useful within matrix programs.
Example -------
. ^matchk A^
Author ------
Nicholas J. Cox, University of Durham, U.K. n.j.cox@@durham.ac.uk