proposition(p)
proposition(q)
proposition(r)

negation(not(X)) :- sentence(X)

conjunction(and(X,Y)) :-
  sentence(X) & sentence(Y)

disjunction(or(X,Y)) :-
  sentence(X) & sentence(Y)

sentence(X) :- proposition(X)
sentence(X) :- negation(X)
sentence(X) :- conjunction(X)
sentence(X) :- disjunction(X)

  istrue(P) :- value(P,true)
  istrue(not(P)) :- ~istrue(P)
  istrue(and(P,Q)) :- istrue(P) & istrue(Q)
  istrue(or(P,Q)) :- istrue(P)
  istrue(or(P,Q)) :- istrue(Q)