Special seek function. (x = xtra)
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 Syntax

         GT_xSeek(xValue, nSeekNum, nOrder, bBlock) --> xRet

 Arguments:

       <xValue>    -  The value to seek for
       <nSeekNum>  -  Option (type of seek (see Description))
       <nOrder>    -  The index order to seek
       <bBlock>    -  A block to eval to get a return value iff
                      required

 Returns:

       <xRet>      -  If nSeekNum == 1, xRet == found()
                   -  If nSeekNum == 2, xRet == found()
                   -  If nSeekNum == 3, xRet == ""
                   -  If nSeekNum == 4, xRet == eval(bBlock)

 Description:

       This function does four separate things dependant on nSeekNum

       nSeekNum    What Happens
       ────────    ────────────────────────────────────────────────────
              1    leave record pointer where found and return found()
              2    return record pointer to previous, return found()
              3    leave record pointer where found and return ''
              4    value = eval(bBlock), go previous rec, return value


 Examples:

        .
        .
     xLookup := DBFILE->(GT_xSeek("KEYS", 4, 1, {|| DESC_40}))
        .
        .

 Source: XSEEK.PRG

 Author:  Andy M Leighton