It is written in fully portable Common Lisp (aside from minor workarounds for implementation-specific issues...), with major unportable bits being handled by external portability libraries.
git clone git://gitorious.org/prepl/prepl.git
prepl is a fork of SBCL's sb-aclrepl module.
Since then, it has been ported to Clozure CL and other Lisps.
Debugging capabilities have been added.
Overall, PREPL does not have Allegro compatibility as a goal, although it still largely keeps its interface.
(asdf:operate 'asdf:load-op 'prepl) (prepl:repl)
The Qt version of Hemlock uses PREPL in its Slave buffers.