This procedure is a simple routine to check assumptions during
programming. If, at any time, an assertion fails to be true, the
program halts with an error message.
ASSERT, assertion [, msg]
assertion: A boolean expression believed to be true
msg: An optional string to print when the assertion fails
The procedure evaluates whether the assertion is true or false. If
true, the program exists quietly. If false, the program issues an
error message and exits
April 2009- Written by Chris Beaumont
July 2011- Added msg input. cnb.