char    temp_string;
int     index;

// Get string length.
index = strlength(haystack);

// There's no working strinlast function, so we'll just
// build our own naive search algorithm here.
//
// We create a temporary string from haystack, starting
// at the far right character. We then use stringinfirst
// to see if our needle is in the temp_string, and if it
// is we now have an index to use for strleft.
//
// If the result is invalid (-1), then we build the
// tempstring from haystack, now one character back, and
// continue until our strinfirst gets a good result.
do
{
    index--;

    // Get the right end of haystack, starting
    // from index.
    temp_string = strright(haystack, index);

} while (strinfirst(temp_string, needle) == -1);


// Now that we have our index pointing to the last
// occurrence of needle in haystack, we can use strleft
// to return the preceding characters.
return strleft(haystack, index);