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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