GET/SET block for screen saver interrupts
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 Syntax

        GT_Interrupt([{|| bInterrupt} ]) -> NIL | bOldInterrupt

 Arguments:

      <bInterrupt> This optional parameter replaces the current interrupt
                   trapping block. This block should return TRUE or FALSE
                   and will be evaluated by other modules in order to
                   determine how the user can interrupt their processing.

 Returns:

      bOldInterrupt - The function returns the currently active interrupt
                      block.  When this block is Evaluated, it will return
                      TRUE or FALSE indicating whether the user has
                      interrupted the current process.

 Description:

      Get-Set function that stores and returns a codeblock which is used to
      interrupt screen savers and other tasks. Screen savers should
      interrogate the functions return to see when they should stop rather
      than using inkey(), mouseclick() etc. The default interrupt block
      is {|| inkey() > 0 }.

 Examples:

      GT_Interrupt( {|| Inkey() > 0 .or. MouseClick() > 0 } )
          --> bOldInterrupt

 Source: GT_INTER.PRG

 Author:  Phillip Hamlyn