Extname and Prefix

I am trying to make use of "qualified field names"  by using Extname on an
externally defined file.  Here is the code, and I can't get it to compile to
V5R1 even though it is in the V5R1 manuals
(http://publib.boulder.ibm.com/iseries/v5r1/ic2924/books/c0925083.pdf page
273).  "The parameter for keyword PREFIX is not valid; keyword is ignored."

I saw some questions on this in the archives which made be believe that is
shouldn't work at V5R1, but why is it in the V5R1 manuals then?


     FLiblPF    IF   E           K DISK    Prefix('Libl.')
     D Libl          E DS                  Qualified ExtName(LiblPF)
      /Free
       *InLR = *On;
      /End-Free 

Here's the file. . .

     A          R LIBL
     A            DIV            3  0       COLHDG('Division')
     A            SUBDIV         3  0       COLHDG('Sub Division')
     A            ID            20A         COLHDG('Unique Id')
     A            SEQ            4  0       COLHDG('Sequence')
     A            LIB           10A         COLHDG('Library')

     A          K DIV
     A          K SUBDIV
     A          K ID
     A          K SEQ