One use case is to ensure that once/1 is safe to use:
term_si(Goal),
once(Goal)
In such cases, Goal is ground and can yield at most one solution,
therefore once/1 does not remove any solutions.
Source: https://stackoverflow.com/questions/20223390/prolog-a-person-is-a-sibling-of-himself
In Scryer Prolog, this is actually not needed, since Scryer Prolog provides
dif/2 in library(dif). However, it is still useful to provide dif_si/2 for two reasons:
1) to more easily port code from systems where only dif_si/2 is available
2) to provide correct disequality in other systems that adopt this library
