documentclass[a4paper]{article}
usepackage[T1]{fontenc}
usepackage[protrusion]{microtype} % protrusion option is actually redundant,
                                   % but I want to be safe!

begin{document}

First of all, ( verb|parfillskip| = mbox{theparfillskip} ).

Secondarily, the main font is ``fontnamethefont'': in this font the
verb|rpcode| of the character~``.'' is therpcodefont`. space and the
verb|lpcode| of the character~``('' is thelpcodefont`( . % paren match )

Now we typeset a paragraph which, while containing the usual verb|parfillskip|
glue, whose value is specified above, ends flush with the right margin because
it contains a verb|hfill| command:hfill
(here!).special{comment:Do you see the kern?}

(Another example for protrusion into the left margin,hfill using
parentheses.)linebreak (We force a left parenthesis at the beginning of a
line.)

begingroup

% Show that protrusion _is_ applied "through" the parfillskip glue:
showboxbreadth = 1000
showboxdepth = 5
showlists

% But if we ask for the rightmarginkern of the last line...
setbox0 = vbox{indent
Now we typeset a paragraph which, while containing the usual verb|parfillskip|
glue, whose value is specified above, ends flush with the right margin because
it contains a verb|hfill| command:hfill (here!).par
globalsetbox1 = lastbox
}
% (...here it is...)
showbox1

endgroup

% ...we don't see any protrusion:
Now, ( verb|rightmarginkern1| = mbox{rightmarginkern1} ).

end{document}