POSR (string, substring)
POSR (string, substring, n)

   Synopsis:
      Returns the position in the string of the last occurence of the whole
         substring

   Notes:
      Specifically, returns the position of the first character of the last
         occurence of the substring in the string. (Axbasic labels the first
         character in the string as 1, the second as 2, and so on.)
      If the substring doesn't occur in the string, 0 is returned. If an empty
         substring is specified, 1 is returned. If the string is empty but the
         substring is not empty, 0 is returned.
      If the third argument is specified, the search begins at the nth character
         in the string and proceeds to the left. POSR(string, substring) is 
         interchangeable with POSR(string, substring, length), where 'length' is
         the length of the string. If the substring is null, the starting
         position is returned.
      If n is not an integer, it is rounded off. An error occurs if n is zero or
         a negative number.
