+$x->{a} ='b' # [ keys %$x ] # '', [ 'a' ], { a => 'b' }
+$x->{a} ='b' # [ keys %$x ] # '', [ 'a' ], { a => 'b' } #
+$x->{a} ='b' # [ keys %$x ] # '', [ 'a' ], { a => 'b' } # +fetch
+$x->{a} ='b' # [ keys %$x ] # '', [ 'a' ], { a => 'b' } # +exists +delete +store
+