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I'm trying to build a simple Prolog SAT solver. My idea is that the user should enter the boolean formula to be solved in CNF (Conjuctive Normal Form) using Prolog lists, for example (A or B) and (B or C) should be presented as sat([[A, B], [B, C]]) and Prolog procedes to find the values for A, B, C. My following code is not working and I'm not understanding why. On this line of the trace Call: (7) sat([[true, true]]) ? I was expecting start_solve_clause([_G609, _G612]]) . Disclaimer: Sorry for the crappy code I didn't even know about Prolog or the SAT problem a few days ago. P.S.: Advice on solving the SAT is welcome. Trace sat([[X, Y, Z], [X, Y]]). Call: (6) sat([[_G609, _G612, _G615], [_G609, _G612]]) ? creep Call: (7) start_solve_clause([_G609, _G612, _G615]) ? creep Call: (8) solve_clause([_G615], _G726) ? creep Call: (9) or(_G725, _G615, true) ? creep Exit: (9) or(true, true, true) ? creep Exit: (8) solve_clause([true], true) ? creep Call: (8) or(_G609, _G612, true) ? creep Exit: (8) or(true, true, true) ? creep Exit: (7) start_solve_clause([true, true, true]) ? creep Call: (7) sat([[true, true]]) ? Prolog Source % Knowledge base or(true, true, true). or(false, false, false). or(true, false, true). or(false, true, true). or(not(true), true, true). or(not(false), false, true). or(not(true), false, false). or(not(false), true, true). or(true, not(true), true). or(false, not(false), true). or(true, not(false), true). or(false, not(true), false). % SAT solver sat([]). sat([Clause | Tail]) :- start_solve_clause(Clause), sat(Tail). % Clause solver start_solve_clause([Var1, Var2 | Tail]) :- solve_clause(Tail, Result), or(Var1, Var2, Result). solve_clause([X | []], Result) :- or(Result, X, true). solve_clause([X | Tail], Result) :- solve_clause(Tail, Result2), or(Result, X, Res

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