title: What is the detail namespace commonly used for
body: <p>In some of the larger projects or libraries (e.g. Eigen), you can see namespaces like <code>internal</code> or <code>detail</code>. I understand what is <code>internal</code> good for, but what is <code>detail</code> commonly used for? Is there any common convention for distributing code into namespaces like that (especially in case of libraries with public interface)?</p>