Prolog: Listing and Debugging Predicates

Listing and debugging predicates.

listing(P): Display predicate P. P may be a predicate name, a structure of the form Name/Arity, or a bracked list of the above.
trace: Turn on tracing.
notrace: Turn off tracing.
spy P: Turn on tracing when predicate P is called. P may be a predicate name, a structure of the form Name/Arity, or a non-bracked list of the above.
nospy P: Turn off spying for P.
nospyall: Turn off all spypoints.
debug: Enable spypoints (allow them to initiate tracing.).
nodebug: Disable spypoints (without removing them).

Control over tracing is very system-dependent, but is probably like this:

<return>: Single-step to next line of trace.
h: Provide help on tracing commands.
s: (On a CALL) Skip over this call.
l: Leap without tracing to the next spypoint.
n: Turn off tracing.
+: Set a spypoint here.
-: Remove the spypoint here.

Trace terminology: CALL is the initial entry to a predicate; EXIT is a successful return; REDO is when it is backed into for another answer; FAIL is when it finds no more solutions. A sequence of calls may be viewed as forming a chain.