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function as3cf_check_required_plugin() {
    if ( class_exists( 'Amazon_Web_Services' ) || !is_admin() || ( defined( 'DOING_AJAX' ) && DOING_AJAX ) ) {
        return;
    }

    require_once ABSPATH . '/wp-admin/includes/plugin.php';
    deactivate_plugins( __FILE__ );

    $msg = sprintf( __( 'Amazon S3 and CloudFront has been deactivated as it requires the <a href="%s">Amazon&nbsp;Web&nbsp;Services</a> plugin.', 'as3cf' ), 'http://wordpress.org/extend/plugins/amazon-web-services/' ) . '<br /><br />';

    if ( file_exists( WP_PLUGIN_DIR . '/amazon-web-services/amazon-web-services.php' ) ) {
        $activate_url = wp_nonce_url( 'plugins.php?action=activate&amp;plugin=amazon-web-services/amazon-web-services.php', 'activate-plugin_amazon-web-services/amazon-web-services.php' );
        $msg .= sprintf( __( 'It appears to already be installed. <a href="%s">Click here to activate it.</a>', 'as3cf' ), $activate_url );
    }
    else {
        $install_url = wp_nonce_url( self_admin_url( 'update.php?action=install-plugin&plugin=amazon-web-services' ), 'install-plugin_amazon-web-services' );
        $msg .= sprintf( __( '<a href="%s">Click here to install it automatically.</a> Then activate it. ', 'as3cf' ), $install_url );
    }

    $msg .= '<br /><br />' . __( 'Once it has been activated, you can activate Amazon&nbsp;S3&nbsp;and&nbsp;CloudFront.', 'as3cf' );

    wp_die( $msg );
}

add_action( 'plugins_loaded', 'as3cf_check_required_plugin' );