input: def check_nested_brackets(input_string):
      '''
      Define a function that receives a string consisting solely of square brackets.
      The function should return True only when there is a valid subsequence of brackets 
      where at least one bracket pair is properly nested.
  
      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 and stack[-1] == '[':
                  stack.pop()
                  if stack:  # if there's still something in the stack, we've seen valid nesting
                      return True
              else:
                  return False
                  
      return False
  