# Pptree(s(NP,VP))

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 Practical 9 Write a predicate pptree/1 that takes a complex term representing a tree, such as s(np(det(a),n(man)),vp(v(shoots),np(det(a),n(woman)))), as its argument and prints a nice and readable output for this tree. pptree(s(NP,VP)) :- write('s('), nl, tab(4), pptree(NP,4), nl, tab(4), pptree(VP,4), write(')'). pptree(det(DET)) :- write('det('), write(DET), write(')'). pptree(n(N)) :- write('n('), write(N), write(')'). pptree(v(V)) :- write('v('), write(V), write(')'). pptree(np(N),I) :- Indent is I+4, write('np('), nl, tab(Indent), pptree(N), write(')'). pptree(np(DET,N),I) :- Indent is I+4, write('np('), nl, tab(Indent), pptree(DET), nl, tab(Indent), pptree(N), write(')'). pptree(vp(V),I) :- Indent is I+4, write('vp('), nl, tab(Indent), pptree(V), write(')'). pptree(vp(V,NP),I) :- Indent is I+4, write('vp('), nl, tab(Indent), pptree(V), nl, tab(Indent), pptree(NP,Indent), write(')'). In the practical session of Chapter 7, you were asked to write a DCG generating propositional logic formulas. The input you had to use was a bit awkward though. The formula ¬ (p → q) had to be represented as [not, '(', p, implies, q, ')']. Now, that you know about operators, you can do something a lot nicer. Write the operator definitions for the operators not, and, or, implies, so that Prolog accepts (and correctly brackets) propositional logic formulas. For example: ?- display(not(p implies q)). not(implies(p,q)). Yes ?- display(not p implies q). implies(not(p),q) Yes :- op(100, fx, not). :- op(200, xfy, and). :- op(300, xfy, or). :- op(400, xfy, implies).

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