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Solve the following problem using Python, implementing the functions described below, one line at a time
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Description:

def ssh_usernames(vm_, opts, default_users=None):
    '''
    Return the ssh_usernames. Defaults to a built-in list of users for trying.
    '''
    if default_users is None:
        default_users = ['root']

    usernames = salt.config.get_cloud_config_value(
        'ssh_username', vm_, opts
    )

    if not isinstance(usernames, list):
        usernames = [usernames]

    # get rid of None's or empty names
    usernames = [x for x in usernames if x]
    # Keep a copy of the usernames the user might have provided
    initial = usernames[:]

    # Add common usernames to the list to be tested
    for name in default_users:
        if name not in usernames:
            usernames.append(name)
    # Add the user provided usernames to the end of the list since enough time
    # might need to pass before the remote service is available for logins and
    # the proper username might have passed its iteration.
    # This has detected in a CentOS 5.7 EC2 image
    usernames.extend(initial)
    return usernames