Return a string where it begins to differ from another
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 Syntax

        GT_StrDiff(<cStr1>, <cStr2>) --> cRet

 Arguments:

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

 Returns:

      cRet     - A string beginning at the position in <cStr2> where
                 <cStr1> begins to differ from <cStr1>

 Description:

      Return a string beginning at the position in <cStr2> where
      <cStr1> begins to differ from <cStr1>. If the two strings are
      identical (or identical upto the last character in <cStr2>)
      the function will return "".

      NOTE:
         invalid parameters will return ""

 Examples:

      ? gt_strDiff("the cat", "the rat")          // prints "rat"
      ? gt_strDiff("the cat", "the ")             // prints ""


 Source: STRDIFF.C

 Author:  Andy M Leighton