IsData validates a number as being ASCII Text.
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 Syntax

        GT_IsData(<nKey>) -> lData

 Arguments:

      <nKey> is the ASCII value to test.

 Returns:

      lData

 Description:

      IsData validates a number as being ASCII Text.

 Examples:

      // Trim out control chars
      IF .NOT. GT_IsData(nKey)
          ? 'Error'
      ENDIF

 Source: GT_ISDAT.PRG

 Author:  Martin Bryant