This example implements a minimal web server which makes it possible
to access the web server's filesystem from a browser and calculate
the sum of numbers entered by the user.

In the filesytem browser, directories are presented as a numbered list
of files.  Regular files ending in ".htm" and ".html" are send back to
the browser as-is, otherwise HTML is dynamically generated to display
the file's content verbatim.

The calculator is an example of web-continuations.  The web server
interacts with the user to obtain a set of numbers to add.  At each
step the sum of the numbers entered up to now is displayed.  Within
each HTML page generated is a form to enter the next number.  This
form has a hidden field which contains a serialized continuation
indicating the continuation within the web server where the number
entered must be returned.  When a thread is created by the web server
to reply to the form submitted, that thread will invoke the
continuation to return to the point where the (now dead) thread
generated the form to obtain the next number.

After starting the server with

                      gsi html http web-server 8000

with your favorite browser visit the URL

                      http://localhost:8000/