GNU Prolog seems not to display unification goals (=
) in the trace. See also this simplified example:
GNU Prolog 1.4.2
By Daniel Diaz
Copyright (C) 1999-2012 Daniel Diaz
| ?- [user].
compiling user for byte code...
f(X) :- X=3.
user compiled, 2 lines read - 182 bytes written, 12539 ms
(266 ms) yes
| ?- trace.
The debugger will first creep -- showing everything (trace)
yes
{trace}
| ?- f(N).
1 1 Call: f(_17) ?
1 1 Exit: f(3) ?
N = 3
yes
{trace}
| ?-
Note that there is no step like X=3
, in contrast to the SWI trace step 7 which CappeliC gave.
So it just means that the next goal after Passenger = ...
fails.