How do I create a custom Context reaction?

I want to create a simple context reaction. Basically, when a condition is met, use drupal_set_message to say "Hello World."

I am using version 7.x-3.0-alpha2 of the Context module, and Drupal 7 Beta 2.

Here is the code I have in my module:

context_helloworld.module:
<?php
function context_helloworld_context_plugins() {
    $plugins = array();
    $plugins['context_helloworld_context'] = array(
    'handler' => array(
        'path' => drupal_get_path('module', 'context_helloworld.context'),
        'file' => 'context_helloworld.context.inc',
        'class' => 'context_helloworld_reaction_helloworld',
        'parent' => 'context_reaction',
        ),
    );
    return $plugins;
}

function context_helloworld_context_registry() {
return array(
    'reactions' => array(
        'bar' => array(
            'title' => t('Hello World Reaction'),
            'plugin' => 'context_helloworld_context',
        ),
    ),
);
}
?>

context_helloworld.context.inc:
<?php
class context_helloworld_reaction extends context_reaction {
    function execute(&$vars = NULL){
        drupal_set_message(t("Hello World from a context reacation."));
    }
}
?>

If I have the module enabled, it gives me a white screen when I try to add a new context.

Of course, without my module enabled, it works fine.

I have been searching for answers on Google, reading the code in the context/plugins, and reading the context documentation.

So, what have I missed that is screwing me up?