Return the position where two strings begin to differ
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 Syntax

        GT_AtDiff(<cStr1>, <cStr2>) --> nPos

 Arguments:

      <cStr1>  - A character string to compare
      <cStr2>  - The string to compare with

 Returns:

      nPos     - The position in <cStr2> where <cStr1> begins to differ

 Description:

      Return the position in <cStr2> where <cStr1> begins to differ.
      If the strings differ in the first character GT_AtDiff() will
      return 1.  If the two strings are identical (or identical upto
      the last character in <cStr2>) the function will return 0.

      NOTE:
         invalid parameters will return -1

 Examples:

      ? gt_atDiff("the cat", "the rat")          // prints 5
      ? gt_atDiff("the cat", "the ")             // prints 0


 Source: ATDIFF.C

 Author:  Andy M Leighton