get 0 4
init: dom`x -> 0
put 0 4
update dom`x to 3
get 0 4
=3:   dom`x -> 3
get 4 8
init: dom`y -> 0
get 0 4
get 0 4
put 4 8
update dom`y to 6
get 4 8
=dom`x+dom`x:   dom`y -> 6