This better expresses the idea that the argument to alwaysTrue is just there to
select an instance. Another option could be to do use a fundep, which seems to
be compatible with the two instances so far.
class Predicate p w | p -> w
LayoutBuilderP is similar to LayoutBuilder (and is based on it), but LayoutBuilderP places windows matching given X.U.WindowProperties.Property (or any other predicate) into one rectangle, instead of fixed number of windows.
