The model used for Flow is "branch on disagree" or "branch on digress". That model is not obvious if you want to mark a comment where you disagree on last reply. Then you must wait for someone to agree on last, before you can disagree on last. Otherwise you must accept a model where posts always are a "disagree on last", but that is neither correct.
Either the intent of the post must be added, or an other reply model must be devised.