ed function invokes the Hemlock editor which is described
in Hemlock User’s Manual and Hemlock Command Implementor’s
Manual. Most users at CMU prefer to use Hemlock’s slave Common Lisp
mechanism which provides an interactive buffer for the
read-eval-print loop and editor commands for evaluating and
compiling text from a buffer into the slave Common Lisp. Since the editor
runs in the Common Lisp, using slaves keeps users from trashing their
editor by developing in the same Common Lisp with Hemlock.