?POP    Pointer off page

          A J,  C  or  R  command  has  been  executed  which
          attempted  to  move  the pointer off the page.  The
          result of executing  one  of  these  commands  must
          leave  the pointer between 0 and Z, inclusive.  The
          characters referenced by a D or m,nX  command  must
          also be within the buffer boundary.