code: def find_pure_symbol(symbols, clauses):
      
      for s in symbols:
          found_pos, found_neg = False, False
          for c in clauses:
              if not found_pos and s in disjuncts(c): found_pos = True
              if not found_neg and ~s in disjuncts(c): found_neg = True
          if found_pos != found_neg: return s, found_pos
      return None, None
docs: Find a symbol and its value if it appears only as a positive literal
      (or only as a negative) in clauses.
      >>> find_pure_symbol([A, B, C], [A|~B,~B|~C,C|A])
      (A, True)