ONSOURCE returns a character string whose value is the contents
of the field that was being processed when the CONVERSION condition
was 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.
>>-ONSOURCE-+----+--------------------------------------------->< '-()-'
If ONSOURCE is used out of context, a null string is returned.
If the source in a failed conversion is a COMPLEX value, then ONSOURCE() will show only the REAL or IMAG half of that value.