input: def check_nested_brackets(input_string):
      '''
      Develop a function to evaluate a string composed solely of square brackets.
      This function should return True if there exists a valid subsequence of brackets
      where at least one bracket is 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_string:
          if char == '[':
              stack.append(char)
          elif char == ']':
              if stack:
                  stack.pop()
  
      return len(stack) < len(input_string) // 2 and stack == [] and input_string.count('[') > 1
  