%CHECKR returns the last position of the string base that
contains a character that does not appear in string comparator.
If all of the characters in base also appear in comparator,
the function returns 0.
The check begins at the starting position and continues to the
left until a character that is not contained in the comparator string
is found. The starting position defaults to the end of the string.
The first parameter must be of type character, graphic, or UCS-2,
fixed or varying length. The second parameter must be the same type
as the first parameter. The third parameter, if specified, must be
a non-float numeric with zero decimal positions.