code: public MemoryFileAttributes addDir( EightyPath dir, Principals principals ) {
  
          if( content.get( dir ).isPresent() ) {
              throw new IllegalArgumentException( "path exists already" );
          }
  
          if( !dir.isAbsolute() || dir.getParent() == null ) {
              throw new IllegalArgumentException( "path not absolute or without parent" );
          }
  
          PathContent parentContent = content.getOrThrow( childGetParent( dir ),
                                                          () -> new IllegalArgumentException( "parent does not exist" ) );
  
          content.put( dir, PathContent.newDir( principals ) );
          parentContent.kids.add( dir );
          parentContent.attis.setLastModifiedTime();
          parentContent.attis.setLastAccessTime();
          return parentContent.attis;
      }
docs: todo default to root ?