Insertion of return values

Prolog variables don't have return values, but often there's a case where a function only works one way, meaning that an unbound variable essentially acts as a return value. While this detracts from the declarative power of Prolog, it certainly reduces the number of temporary variables hanging around programs.

So, to shorten the pattern cut implementations and reduce the number of temporary variables we use the notation <-
eg. We can rewrite bar(X), foo(X,c) as foo(bar(<-),c)

In useful terms, one nice rewrite we could have is

statement(Ta,CallA), statement(Tb, CallB), precedes(Ta,Tb)
rewrites to a much shorter and intuitve
precedes(statement(<-,CallA), statement(<-,CallB))

More extreme examples that certainly cut down on the number of inserted variables but potentially at the cost of legibility could be
dominates(a,b) = parentMethod(a,Ta), body(Ta,Tb), start(Tb, Tc), between(Tc,b,a,true)
which shortens to
dominates(a,b) = between(start(body(parentMethod(a,<-),<-),<-),b,a,true)

Its the type of predicate that's handy to have in your arsenal but its good to be aware of its impact on readability