POS (string, substring)
POS (string, substring, n)

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

   Notes:
      Specifically, returns the position of the first character of the first
         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 a null
         substring is specified, 1 is returned. If the string is null but the
         substring is not null, 0 is returned.
      If the third argument is specified, the search begins at the nth character
         in the string and proceeds to the right. POS(string, substring, 1) is
         interchangeable with POS(string, substring). If the substring is empty,
         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.
