ONWCHAR returns a widechar(1) string containing the widechar that
caused the CONVERSION condition to be raised. It is in context in
an ON-unit (or any of its dynamic descendants) for the CONVERSION
condition or for the ERROR or FINISH condition raised as the implicit
action for the CONVERSION condition.
>>-ONWCHAR-+----+----------------------------------------------><
'-()-'
If the ONWCHAR built-in function is used out of context, a widechar
blank is returned.