class_docstr: Represents a single level in the DPLL algorithm, and contains
  enough information for a sound backtracking procedure.
class_docstr_tok: ["Represents","a","single","level","in","the","DPLL","algorithm",",","and","contains","enough","information","for","a","sound","backtracking","procedure","."]
code: def __init__(self, decision, flipped=False):
      self.decision = decision
      self.var_settings = set()
      self.flipped = flipped
  
imports: from collections import defaultdict
  from heapq import heappush, heappop
  from sympy.core.sorting import ordered
  from sympy.assumptions.cnf import EncodedCNF
  from sympy.logic.algorithms.lra_theory import LRASolver
label_desc: Use image node_id 1 to create a new Level object  with example: obj = Level(decision, flipped)