Print passes through ports of specified predicates. Pred is a,
possible partial, specification of a predicate as it is also used be
spy/1 and similar predicates. Where a full predicate specification
is of the shape
Module:Name/Arity (or `//Arity for non-terminals),
both the module and arity may be omitted in which case Pred refers
to all matching predicates. PortSpec is either a single port
redo), preceded with
- or a
list of these. The predicate modifies the current trace
specification and then installs a suitable wrapper for the predicate
using wrap_predicate/4. For example:
% lists:append/2: [all]
% lists:append/3: [all]
% append/1: [all]
?- append([a,b], [c], L).
T  Call: lists:append([a, b], [c], _18032)
T  Call: lists:append([b], [c], _19410)
T  Call: lists:append(, [c], _20400)
T [28 +0.1ms] Exit: lists:append(, [c], [c])
T [19 +0.2ms] Exit: lists:append([b], [c], [b, c])
T [10 +0.5ms] Exit: lists:append([a, b], [c], [a, b, c])
L = [a, b, c].
?- trace(append, -all).
% lists:append/2: Not tracing
% lists:append/3: Not tracing
% append/1: Not tracing
The text between  indicates the call depth (first number) and for
all ports except the
call port the wall time since the start
(call port) in milliseconds. Note that the instrumentation and print
time is included in the time. In the example above the actual time
is about 0.00001ms on todays hardware.