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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 roles_add(user, role):
 """Add user to role."""

    user, role = _datastore._prepare_role_modify_args(user, role)
    if user is None:
        raise click.UsageError('Cannot find user.')
    if role is None:
        raise click.UsageError('Cannot find role.')
    if _datastore.add_role_to_user(user, role):
        click.secho('Role "{0}" added to user "{1}" '
                    'successfully.'.format(role, user), fg='green')
    else:
        raise click.UsageError('Cannot add role to user.')