Set a number of flags to FALSE in a bit flag string.
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 Syntax

        GT_ClrFlag(<cFlagString>,[<nStart>],[<nEnd>]) --> cFlagString

 Arguments:

      <cFlagString> is a bit flag string created with GT_NewFlag()

      <nStart> is the starting flag. This is an optional numeric value.
      If not supplied it defaults to 1.

      <nEnd> is the ending flag. This is an optional numeric value. If
      not supplied it defaults to <nStart>.

 Returns:

      The bit map string with the new flag settings.

 Description:

      GT_ClrFlag() is used to turn flags within the flag string off.

 Examples:

      cFlags := GT_NewFlag(20)   // Create a bit flag string for 20
                                 // logical values.

      // Now, turn them all on.

      cFlags := GT_SetFlag(cFlags,1,20)

      // Now set flags 10 to 15 to false.

      cFlags := GT_ClrFlag(cFlags,10,15)

      // And set flag 18 to false.

      cFlags := GT_ClrFlag(cFlags,18)

      // And set flag 1 to false.

      cFlags := GT_ClrFlag(cFlags)

 Source: BITFLAGS.C

 Author:  Dave Pearson