Mystery php

I am working with the node_access_example.module
It gives the following database definition

CREATE TABLE node_access_example (
nid int(10) unsigned NOT NULL default '0' PRIMARY KEY,
private int,
KEY `node_example_nid` (nid)
)

I have used the following to create the table but I am sure it is incorrect because I don't understand the syntax above.

function node_access_example_schema() {
  $schema['annotations'] = array(
    'description' => t('Stores node annotations that users write.'),
    'fields' => array(
      'nid' => array(
        'type' => 'int',
        'unsigned' => TRUE,
        'not null' => TRUE,
        'default' => 0,
        'primary key' => 'private int',
        'key' => `node_example_nid`,
        ),
      ),    
  );
  return $schema;
}

Can anybody tell me the right way to do this please.