A demo shows it better than words: http://subtextual.org/demo1.html
And a newer demo, although it does not show the call graph expansion: http://subtextual.org/subtext2.html
A demo shows it better than words: http://subtextual.org/demo1.html
And a newer demo, although it does not show the call graph expansion: http://subtextual.org/subtext2.html