FitNesse. UserGuide. FitLibraryUserGuide. DoFixture.
WhenActionsFail [add child]
Let's look at what happens when an action fails.

ChatStart

connect user sarah

user sarah creates fit room

There should be no occupants in the "fitNesse" room:

check occupants fit 0

Sarah can't enter an unknown room:

user sarah enters unfit room

We can expect that, by putting reject in the first cell:

reject user sarah enters unfit room

and an unknown user can't create a room:

reject user george creates unfit room

Sarah hasn't entered the room, so she can't be in there:

users in room fit
name
sarah

Here's a DoFixtureSummary.