Docs on systems have a few issues

[WetDesertRock]

The docs say something like:

 function system:preProcess(entities, dt) -- Called before iteration.
 function system:process(entities, dt) -- Process each entity.
 function system:postProcess(entity, dt) -- Called after iteration.

Really should be:

 function system:preProcess(dt) -- Called before iteration.
 function system:process(entity, dt) -- Process each entity.
 function system:postProcess(dt) -- Called after iteration.

[bakpakin]

Just fixed and uploaded docs.