After you've implemented the code in sat_solver.pl,
answer the questions below.  This portion is
purely bonus worth up to 15% of the assignment;
you do not need to complete this for full credit.

1.) The SAT solver from assignment 1 used a list, but
    no lists are used in the Prolog-based SAT solver.
    How is this solver able to avoid using lists?




2.) The SAT solver from assignment 1 used a hash table
    (i.e., a mapping) from variables to Boolean values.
    However, no hash tables or maps are used in the
    Prolog-based SAT solver.  How is this solver able
    to avoid using hash tables / maps?




3.) The function you implemented for assignment 1
    returned either a hash table or null, depending
    on whether or not the solver was dealing with
    a satisfiable or unsatisfiable formula.  The
    Prolog-based SAT solver, however, doesn't exactly
    return anything, but it still produces values.
    How is the Prolog-based SAT solver producing values?
    (As a hint, this question is connected to the
    previous question.)