Check Key for Upper/Lower case, Control or Alt Letter
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 Syntax

         GT_checkKey(<nKVal>, <cChkLetter>) --> lRet

 Arguments:

       <nKVal>       - the inkey() value
       <cChkLetter>  - the letter to check for

 Returns:

       lRet          - .T. if cChkLetter is pressed

 Description:

       This function checks an inkey value against a letter to see
       if that letter has been pressed.
       Detects lower and uppercase, CTRL letter and ALT letter.

 Examples:

 Source: CHKKEY.PRG

 Author:  Andy M Leighton