input: def check_nested_brackets(input_str):
      '''
      Write a function that accepts a string consisting solely of square brackets.
      This function should return True if there exists a valid bracket subsequence
      where at least one bracket is properly nested within another.
  
      check_nested_brackets('[[]]') ➞ True
      check_nested_brackets('[]]]]]]][[[[[]') ➞ False
      check_nested_brackets('[][]') ➞ False
      check_nested_brackets('[]') ➞ False
      check_nested_brackets('[[][]]') ➞ True
      check_nested_brackets('[[]][[') ➞ True
      '''
output:     stack = []
      for char in input_str:
          if char == '[':
              stack.append(char)
          elif char == ']':
              if stack:
                  stack.pop()
              if stack and stack[-1] == '[':
                  return True
      return False