SUB_BIN   This is a new device driver being shipped with later versions of the
          Rebel code. It adds another device to the QL called SUB. short for
          SUB-directory, or SUBstitute. It behaves like a normal directory
          device, but allows a sub-directory to pretend to be a totally
          separate drive. It is configurable with the standard QJUMP CONFIG
          program to set your favourite 8 sub-directories. I wrote this to
          get around Quill not understanding filenames > 8 characters, and
          since others found it useful, I extended it to cope with wildcards
          and directory access.

          There are four new keywords associated with it...

          SUB_USE name makes the SUB device respond to another name eg.
          SUB_USE flp means the SUB device will call itself FLP in future.
          Useful for emulation. SUB_USE by itself resets the name to the
          configured default.

          SUB_USE n,name sets a sub-directory string. n should be a number
          from 1 to 8. SUB_USE 2,fred will set SUB2_ to be DRVx_fred_
          See examples below.

          SUB_USE$ is a function returning the current name of the SUB device.
          This will be SUB unless you changed it.
          SUB_USE$(n) where n is a number from 1 to 8 returns the associated
          sub-directory string.

          SUB_DRV name changes the drive the SUB driver looks on for it's
          sub-directories.
          SUB_DRV$ is a function returning the current drive used.

          Examples.
          With the defaults configured as supplied, DIR sub1_  will give a
          directory of win1_work_

          PRINT SUB_USE$         returns SUB
          PRINT SUB_USE$(1)      returns work_
          PRINT SUB_DRV$         returns WIN1

          SUB_DRV flp2 tells the sub device to look on flp2 in future, so now
          DIR SUB1_ gives a directory of flp2_work_

          SUB_USE 1,asm_temp_ now changes the directory used, so DIR sub1_
          now gives a directory of flp2_asm_temp_

          Uses.
          Most useful for emulation, so...
          FLP_USE fdk    to change the floppy driver name
          SUB_USE flp    to make the SUB device pretend it's called FLP
          and if your defaults need changing...
          SUB_DRV win1 : SUB_USE 2,quill_docs_

          Now if you DIR FLP1_ you will list all files in the directory
          WIN1_quill_docs_, and loading a file within Quill as 'flp1_test_doc'
          will actually load 'win1_quill_docs_test_doc', which is an illegal
          name as far as Quill is concerned. If you really want to load a file
          from flp1 called 'test_doc', call it 'fdk1_test_doc' (which is why we
          changed the floppy driver name first).