|If a reference link is followed, the current node is usually replaced by the node linked to.
If a note link is follwed, then generally a pop up window is created in which the new node is displayed, keeping
the current node in view in the main browser window. An expansion link can be used to reveal more information, to branch out,
in the original space of the link. The link can conversely be collapsed, leaving just the original
link. A command link, if followed, causes an action to be performed, such as downloading a file, or running an executable file.|