input: Write a python function to check if a virtual environment is active by checking the presence of the environment variable 'VIRTUAL_ENV'.
  assert is_virtualenv_active() == True, "The virtual environment should be active when 'VIRTUAL_ENV' is set."
  assert not is_virtualenv_active(), "The virtual environment should not be active when 'VIRTUAL_ENV' is not set."
output: def is_virtualenv_active():
      import os
      return 'VIRTUAL_ENV' in os.environ