VMS Help NCS Command, Command Qualifiers, /COMPRESS  *Conan The Librarian (sorry for the slow response - running on an old VAX)

  /COMPRESS[=(option[,...])]

   Effectively recovers disk space previously  occupied  by  definitions
   that have been deleted from the library by creating a new compressed
   version of the library.    If you do not explicitly specify an input
   library  file  using  the  /LIBRARY  qualifier,  NCS  compresses the
   current  version  of  SYS$LIBRARY:NCS$LIBRARY.NLB.  If  you  do  not
   explicitly  specify  an  output  library  file  using  the   /OUTPUT
   qualifier,     NCS     writes    the    compressed    library     to
   SYS$LIBRARY:NCS$LIBRARY.NLB.

   Options include BLOCKS:n, HISTORY:n, KEEP, KEYSIZE:n, MODULES:n.