ansi 9.6.1.2275
<EXCEPTION>
raise an exception - it will adjust the depth
of all stacks and start interpreting at the point
of the latest CATCH
if n is null nothing happens, the -1 (ie. FALSE )
is the raise-code of ABORT - the other codes
are implementation dependent and will result in
something quite like ABORT
reference: p4_throw in ../src/xception.c:0055, export CO THROW