From the getting started tutorial,[1] I notice that (certain) functions appear to have the method "system" added to them. For example:<p>> Note the hello_world.system() function call. This is a "trait extension method" that converts the hello_world function into the System type. ...<p>Later follows an example where a function previously defined as add_people appears to have a method called "system", allowing expressions like this:<p><pre><code> add_people.system()
</code></pre>
I don't see any macros on the function definitions, so where does this come from. Also, what's the purpose of adding the system method to functions?<p>[1] <a href="https://bevyengine.org/learn/book/getting-started/ecs/" rel="nofollow">https://bevyengine.org/learn/book/getting-started/ecs/</a>