We don't always want to keep typing into the top-level interpreter. How can we implement other forms of interaction?
Prolog has a number of built-in predicates. You have seen
is and \=. There are also predicates for input and output.
These belong to the non-logical part of Prolog: they have no
logical interpretation because they change the world. To use them, you
have to know how Prolog is executed. Note: Prolog programmers always call
them predicates, even though they have no logical interpretation.