The compositor accepts one change to one output at a time. This crate compares a snapshot with a layout and gives the smallest group of the changes. Each change belongs to one of four stages, and the stages keep this order: 1. Turn on the outputs. 2. Set the mode, the scale factor and the transform. 3. Set the position. 4. Turn off the outputs. The last stage comes last for safety. An order that turns off an output first can leave the computer with no output, and a user cannot then correct the layout. The action type of the "niri-ipc" crate has no comparison. The crate therefore keeps a change type of its own. The program compares that type, shows it to a user, and then converts it into an action. The crate also examines a layout. Two conditions are errors: a layout that turns off all the outputs, and a layout that gives a mode which the monitor does not have. An overlap between two outputs and a space between two groups are warnings, because the compositor accepts them.