ONGSOURCE returns a graphic string containing the DBCS character
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 a CONVERSION condition.
>>-ONGSOURCE-+----+-------------------------------------------->< '-()-'
If the ONGSOURCE built-in function is used out of context, a null GRAPHIC string is returned.