┌ Another person built gold block(Owner is another person) on my iron block. and I built iron block(Owner is me) on gold block(Owner is another person)
3 floor:             ● <  Iron block(Owner is me)
2 floor:             O < Gold block(Owner is another person)
1 floor:             ● < (Owner is me)
ㅁㅁㅁㅁㅁㅁㅁㅁㅁㅁㅁㅁㅁㅁㅁㅁㅁㅁㅁ < Dirt block

If I break iron block on 1 floor, Two blocks fall down. But I can't break iron block on iron block. this block owner is changed to another person. and I can break gold block. this block owner is changed to me.
2 floor:             ● < Iron block
1 floor:             O < Gold block
ㅁㅁㅁㅁㅁㅁㅁㅁㅁㅁㅁㅁㅁㅁㅁㅁㅁㅁㅁ