Secondary segments
Since a router state is a tree, and the URL is nothing but a serialized state, the URL is a serialized tree. In all the examples so far every segment had only one child. For instance in /inbox/33 the 33 segment is a child of inbox, and inbox is a child of the / root segment. We called such children primary. Now look at this example:
/inbox/33(popup:message/44)
Here the root has two children inbox and message:
The router encodes multiple secondary children using a //.
/inbox/33(popup:message/44//help:overview)
If some other segment, not the root, has multiple children, the router will encode it as follows:
/inbox/33/(messages/44//side:help)
