'Handle the unsetting of a literal for the VSIDS heuristic.
Examples
>>> from sympy.logic.algorithms.dpll2 import SATSolver
>>> l = SATSolver([{2, -3}, {1}, {3, -3}, {2, -2},
... {3, -2}], {1, 2, 3}, set())
>>> l.lit_heap
[(-2.0, -3), (-2.0, 2), (-2.0, -2), (0.0, 1), (-2.0, 3), (0.0, -1)]
>>> l._vsids_lit_unset(2)
>>> l.lit_heap
[(-2.0, -3), (-2.0, -2), (-2.0, -2), (-2.0, 2), (-2.0, 3), (0.0, -1),
...(-2.0, 2), (0.0, 1)]'

def _vsids_lit_unset(self, lit):

    var = abs(lit)
    heappush(self.lit_heap, (self.lit_scores[var], var))
    heappush(self.lit_heap, (self.lit_scores[(- var)], (- var)))
