code: def find_pure_symbol(symbols, unknown_clauses):
      """
      Find a symbol and its value if it appears only as a positive literal
      (or only as a negative) in clauses.
  
      >>> from sympy.abc import A, B, D
      >>> from sympy.logic.algorithms.dpll import find_pure_symbol
      >>> find_pure_symbol([A, B, D], [A|~B,~B|~D,D|A])
      (A, True)
  
      """
      for sym in symbols:
          found_pos, found_neg = False, False
          for c in unknown_clauses:
              if not found_pos and sym in disjuncts(c):
                  found_pos = True
              if not found_neg and Not(sym) in disjuncts(c):
                  found_neg = True
          if found_pos != found_neg:
              return sym, found_pos
      return None, None
  
file_path: sympy/sympy/logic/algorithms/dpll.py
imports: from sympy.core.sorting import default_sort_key
  from sympy.logic.boolalg import Or, Not, conjuncts, disjuncts, to_cnf, to_int_repr, _find_predicates
  from sympy.assumptions.cnf import CNF
  from sympy.logic.inference import pl_true, literal_symbol
label_desc: Use image node_id 7 for calling a global function with example usage: find_pure_symbol(symbols, unknown_clauses) and returns: None,  None, sym,  found_pos