in Eden editor
1) open debug console, execute
uinamespace setVariable ["first_EH", (add3DENEventHandler ["OnMissionNew",{systemChat "first EH"}])];
uinamespace setVariable ["second_EH", (add3DENEventHandler ["OnMissionNew",{systemChat "second EH"}])];
uinamespace setVariable ["third_EH", (add3DENEventHandler ["OnMissionNew",{systemChat "third EH"}])];
uinamespace setVariable ["forth_EH", (add3DENEventHandler ["OnMissionNew",{systemChat "forth EH"}])];
2) Press "new mission button" ---> messages displayed
3) open debug console and execute
remove3DENEventHandler ["OnMissionNew",(uinamespace getVariable "first_EH")];
remove3DENEventHandler ["OnMissionNew",(uinamespace getVariable "second_EH")];
remove3DENEventHandler ["OnMissionNew",(uinamespace getVariable "third_EH")];
remove3DENEventHandler ["OnMissionNew",(uinamespace getVariable "forth_EH")];
4) Press "new mission button" ---> messages displayed again, while they should not.