text: def checkio(expression):
      stack = []
      dict_b = {"{": "}", "[": "]", "(": ")"}
      for letter in expression:
          if letter in dict_b.keys():
              stack.append(letter)
          if letter in dict_b.values():
              if not (stack and dict_b[stack.pop()] == letter):
                  return False
      return not stack
  
  
  def test_function():
      assert checkio("((5+3)*2+1)") == True, "Simple"
      assert checkio("{[(3+1)+2]+}") == True, "Different types"
      assert checkio("(3+{1-1)}") == False, ") is alone inside {}"
      assert checkio("[1+1]+(2*2)-{3/3}") == True, "Different operators"
      assert checkio("(({[(((1)-2)+3)-3]/3}-3)") == False, "One is redundant"
      assert checkio("2+3") == True, "No brackets, no problem"
  
  if __name__ == '__main__':
      test_function()
  