input: Write a Python function to set and get the backup library used in a circuit, and test with assert statements.
  ```python
  def set_backup_lib(lib):
      global backup_lib
      backup_lib = lib
  
  def get_backup_lib():
      return backup_lib
  
  assert get_backup_lib() is None  # Initially, the backup library should be None.
  set_backup_lib("test_lib")
  assert get_backup_lib() == "test_lib"  # After setting, the backup library should be "test_lib".
output: def set_backup_lib(lib):
      global backup_lib
      backup_lib = lib
  
  def get_backup_lib():
      return backup_lib
  
  # Initialize backup_lib to None
  backup_lib = None
  
  assert get_backup_lib() is None  # Initially, the backup library should be None.
  set_backup_lib("test_lib")
  assert get_backup_lib() == "test_lib"  # After setting, the backup library should be "test_lib".