'Build a clause representing the fact that at least one decision made
so far is wrong.
Examples
>>> from sympy.logic.algorithms.dpll2 import SATSolver
>>> l = SATSolver([{2, -3}, {1}, {3, -3}, {2, -2},
... {3, -2}], {1, 2, 3}, set())
>>> next(l._find_model())
{1: True, 2: False, 3: False}
>>> l._simple_compute_conflict()
[3]'

def _simple_compute_conflict(self):

    return [(- level.decision) for level in self.levels[1:]]
