XPcheckSum()
  Perform a simple CheckSum on a Memo/Char variable
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   Function:   XPcheckSum()

               XPcheckSum() is a function that will count all the values
               of the characters in a char/memo var and return a number
               indicating the count (modulo 2^16). It is a simple way of
               validating data, it won't fool everybody, but it can always
               came in handy. XPcheckSum() is written in Assembly, and therefore
               quite fast, no delay will be experienced on even big Memo
               variables.

   Syntax:     XPcheckSum(cVar) --> nCheck

   Arguments:  <cVar> is the variable to check, and Char/Memo is ok.

   Returns:    <nCheck> is the result of the checkSum performed.
               This could be used to check whether a Data file has
               been altered, or whatever.

   Usage:      if XPcheckSum(MemoRead("MSG.DAT")) <> nSavedCheck
                    ? "you have been messing with MSG.DAT"
                    QUIT
               end