Function:   SetDrive()

               This function provides some control over the
               Acces of logical drives from DOS. Using SetDrive()
               one can set a drive letter as being the One that
               has last accessed a Drive. Needs DOS 3.20!.

   Syntax:     SetDrive([<nDisk>]) --> nLastAssigned

   Arguments:  <nDisk> is the drive to Set, 0 or none indicates
               current drive, 1=A: etc.. Note that the coding
               begins with 0 = default and 1=A, contrary to some
               other Disk functions.

   Returns:    a Numeric, 0 if the specified disk drive has only
               One letter assigned to it, if Non Zero, the
               function returns the highest possible drive Letter
               the specified drive can be accessed with.

   Usage:      This function provides a documented and DOS
               supported way to avoid the 'Insert Disk ..'
               Message. A possible way is to allways execute
               SetDrive(i+1) before Executing SetDisk(i), but one
               can also check if drive X: was last accessed using
               GetDrive() (it should be 0 or refer to X:), if
               Not, Your program could generate a prompt asking
               for a drive to be entered, and then execute
               SetDrive() to avoid DOS' message.
               The file EXPDEMO.PRG has a good example of usage
               of this function, in the ChangeDrive() function.