code: def prop_symbols(x):
      "Return a list of all propositional symbols in x."
      if not isinstance(x, Expr):
          return []
      elif is_prop_symbol(x.op):
          return [x]
      else:
          return list(set(symbol for arg in x.args
                          for symbol in prop_symbols(arg))f prop_symbols(x):
      "Return a list of all propositional symbols in x."
      if not isinstance(x, Expr):
          return []
      elif is_prop_symbol(x.op):
          return [x]
      else:
          return list(set(symbol for arg in x.args
                          for symbol in prop_symbols(arg)))
docs: Return a list of all propositional symbols in x.