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3

The above rule in prefix form



:-(happy(X)
rich(X)).

[-5pt]

But what about



happy(X):-



healthy(X)




wealthy(X)




wise(X).

Trying to rewrite in prefix form:



:-(happy(X)
whatgoeshere?).

[-5pt]

Note that the comma ` ' in this expression is an argument separator. In the definition of happy/1 above the commas are read as ``and''.

Yes



healthy(X)
wealthy(X)
wise(X).

[-5pt]

is also a compound term with principal functor






[-5pt]

and arity 2. Since we have to represent three subgoals and the arity of ` ' is 2 we again have a nested compound term. The correct prefix form for the example is:



'
'(healthy(X)
'
'(wealthy(X)
wise(X))).

Note: try the goal display((healthy(X) wealthy(X) wise(X))) to see the ``truth''. Also note that for a reason as yet unexplained you need an extra pair of brackets around the goal you want printed via display/1.

Here is the tree:

Paul Brna
Mon May 24 20:14:48 BST 1999