  Permission to use, copy, modify and distribute the DocBook DTD and
  its accompanying documentation for any purpose and without fee is
  hereby granted in perpetuity, provided that the above copyright
  notice and this paragraph appear in all copies.  The copyright
  holders make no representation about the suitability of the DTD for
  any purpose.  It is provided "as is" without expressed or implied
  warranty.

  If you modify the DocBook DTD in any way, except for declaring and
  referencing additional sets of general entities and declaring
  additional notations, label your DTD as a variant of DocBook.  See
  the maintenance documentation for more information.

