{Joseph Wright}
{Supporting backends in expl3}
{The backend in \TeX\ is responsible for the parts of producing output 
that \TeX\ doesn't know about, for example colour, image inclusion and 
hyperlink creation. Each backend has its own syntax and range of 
supported concepts, so at the macro level there needs to be the 
appropriate code to `talk' to the backend. In expl3, we have developed a 
consistent set of backend support files, based on the experience of 
\AllTeX\ developers over 30+ years of working with these backends. Here, 
I will look at the history of backend abstraction and the model used in 
expl3.}