In the default GNU Emacs, started with `emacs -Q -D', that sentence
appears as:
See `http://www.rattlesnake.com/notions/Choice-and-Constraint.html',
which is intended as a description of `what is' ...
(with the URL in blue and underlined).
In the default GNU Emacs, when I move the mouse over the URL, the
background changes to a light blue green that is called
`darkseagreen2'. When I click a mouse cursor on the URL, the message
area says both:
mouse-1: go to this URL
Loading thingatpt...done
(which I quoted out of the *Messages* buffer) but nothing happens.