determines where (in S1) the first time one of the characters of S2 appear. The result is the length of a string part of S1 where none of the characters of S2 do appear (not counting the trailing #0 and starting with position 0 in S1).
This function does the same as StrRNScanW but uses S1 in reverse order. This means S1 points to the last character of a string, is traveresed reversely and terminates with a starting #0. This is useful for parsing strings stored in reversed macro buffers etc.