Documents within a virtual class do not have the same system of access
control as \link{code}{those outside classes}.

Virtual class documents are not accessible from outside the class where they
are created. But it may contain elements (blocks) of documents outside the
classes. To do so, it is enough to use the commands
\link{link}{\link} or \link{embed}{\embed}.

Only the supervisor of the class can create a document and modify its
properties. He may decide on the access right to the document by
participants of the class (modifiable among properties of the document):

- Neither read no write. Participants totally ignore the existance of the
document. This is in particular the desired mode when the document is under
preparation.

- Read but not write. This is the usual acccess mode of a course material.

- Read and write. In this case the document can be freely modified by any
participant of the class, and there will be no trace of who has done what.
Useful for a chatting space.

Some variables are pre-defined for a document in a virtual class:
\\class_code, \\user_login, \\user_firstname, \\user_lastname

