'VSIDS Heuristic Calculation
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_calculate()
-3
>>> l.lit_heap
[(-2.0, -2), (-2.0, 2), (0.0, -1), (0.0, 1), (-2.0, 3)]'

def _vsids_calculate(self):

    if (len(self.lit_heap) == 0):
        return 0
    while self.variable_set[abs(self.lit_heap[0][1])]:
        heappop(self.lit_heap)
        if (len(self.lit_heap) == 0):
            return 0
    return heappop(self.lit_heap)[1]
