The Find First Character in a Set of Characters routine searches
    a string, comparing each character to the characters in a
    specified set of characters. The string is searched character
    by character, from left to right. STR$FIND_FIRST_IN_SET returns
    the position of the first character in the string that matches
    any of the characters in the selected set of characters.

    Format

      STR$FIND_FIRST_IN_SET  source-string ,set-of-characters