Sweep highlights holes in Prolog buffer by default so you can easily identify missing terms.
Whether to highlight holes in Sweep Prolog mode buffers with a
dedicated face. By default, this is set to t.
When the user option sweeprolog-highlight-holes is set to
non-nil, Sweep highlights holes in Prolog buffers with a
dedicated face to make them easily distinguishable from regular Prolog
variables. Hole highlighting is enabled by default, to disable it
customize sweeprolog-highlight-holes to nil.