Science fiction stories

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## Science fiction stories

Start by loading knowledge base `spin` with the command `library(spin).` It defines a predicate with no arguments at all, also called `spin`. Since all the predicates you have seen so far have had arguments, you may find it hard to see how argument-less predicates could be any use. After all, the truth value of a predicate depends on its arguments; surely, if it has none, then it must be always true or always false? However, as we shall see in Lesson 7, such predicates can be useful. There is more to Prolog than logic.

Start by calling `spin` a few times, by asking the question

`    spin.`

Now, have a look at the knowledge base. It contains a number of unconditional facts whose predicate is `element`, with three arguments. How do they describe the stories `spin` generates? Sketch the process by which you think `spin` operates.

Note incidentally that the arguments to `element` are almost all quoted atoms. We need to quote them because they contain spaces, capitals and other funny characters that would not make sense in unquoted atoms.

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Jocelyn Paine
Tue Jun 4 17:58:48 BST 1996