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- add missing \label
- add errormessage generation to lonhelp.pm in case it does not find a label for
   the reference

- This should fix #3016, and make it easier to find these errors in the fututre

\label{Guts_Apache} The standard mode in which the Apache web server is used is that a URL corresponds to some static file on the file system, which then more or less gets sent out as-is. Slight deviations from that simple principle are however already the directory listing function, virtual servers, and the cgi-bin directory. In the latter case, Apache executes the file in a new process and provides both the input to the process in the environment, and routes the output to the client. Other deviations are the error messages. In a more general view, URLs to Apache are URIs (Uniform Resource Identifiers), which may are may not correspond to a physical file, which in turn may or may not be sent out as-is. As a request for a URI gets sent to the Apache server, it goes through several phases. At each phase ({}``stage'') along the process, handler modules can be defined to deal with the request. Details about these phases are given in {}``Writing Apache Modules with Perl and C'' from O'Reilly (the {}``Eagle book'') in Chapter 3 (page 60). These handler modules are not like cgi-scripts executed in a separate process, but are dynamically linked into the Apache child processes themselves - they run inside of Apache. The mod\_perl mechanism in addition links the Perl interpreter into the Apache child process. Modules are pre-interpreted ({}``compiled'') by the Perl interpreter when they are first loaded, and the pre-interpreted code stays permanently in the memory allocated to that Apache child process. The result is a significant speed-up, and through the flexible mechanism of module registration and different stages of the process, a high degree of customizability. LON-CAPA does \emph{not} use Apache::Registry, and so in addition avoids the unnecessary overhead of emulating a cgi-environment within the handlers. Instead, it makes direct use of the Apache Perl API (Chapter 9, Eagle book). If you intend to add new capabilities to LON-CAPA that require new handlers, as opposed to tweaking current ones, we recommend buying a copy of {}``Writing Apache Modules with Perl and C''; it is not strictly necessary but it is the clearest description of what is going on in the Apache server, and without it you will be challenged to understand everything that is occurring in the server.