PceEmacs showing a coloured Prolog file, the summary description of the current goal and a just-set break-point. Breakpoints are set at a specific call in a specific clause (opposite to traditional spy-points that trap the tracer on any call to a specified predicate). As execution reaches the break-point, the source-level debugger is activated.

PceEmacs is started using ?- emacs. or ?- emacs(File). It can be made the default editor for the edit/0 and edit/1 commands by setting the Prolog flag editor. For example by adding the following to .plrc (pl.ini on MS-Windows):

:- set_prolog_flag(editor, pce_emacs).