A declaration
:- mode ModeSpec, ..., ModeSpec.
where each ModeSpec is a mode spec, has no effect whatsoever, but is accepted for compatibility reasons. In some Prologs, this declaration helps reduce the size of the compiled code for a predicate, and may speed up its execution. Unfortunately, writing mode declarations can be error-prone, and since errors in mode declaration do not show up while running the predicates interpretively, new bugs may show up when predicates are compiled. However, mode declarations may be used as a commenting device, as they express the programmer's intention of data flow in predicates.