xref_source/1 error

[pmoura]

$ yap
% YAP 6.5.0-31743139 (compiled  2019-04-07T01:08:28@MacBook-Pro-3)
% database loaded from /usr/local/lib/Yap/startup.yss
?- use_module(library(lists)).
reconsulting library(lists)...
reconsulted /usr/local/share/Yap/lists.yap in module lists, 7 msec 65432 bytes
true
?- use_module(library(prolog_xref)).
reconsulting library(prolog_xref)...
 % reconsulting library(debug)...
  % reconsulting library(hacks)...
  % reconsulted /usr/local/share/Yap/hacks.yap in module yap_hacks, 1 msec 8216 bytes
 % reconsulted /usr/local/share/Yap/debug.pl in module prolog_debug, 6 msec 55632 bytes
 % reconsulting library(operators)...
 % reconsulted /usr/local/share/Yap/operators.pl in module prolog_operator, 2 msec 14824 bytes
 % reconsulting library(shlib)...
 % reconsulted /usr/local/share/Yap/shlib.pl in module shlib, 4 msec 33816 bytes
 % reconsulting library(prolog_source)...
 % reconsulted /usr/local/share/Yap/prolog_source.pl in module prolog_source, 3 msec 21632 bytes
 % reconsulting library(option)...
 % reconsulted /usr/local/share/Yap/option.pl in module swi_option, 4 msec 35592 bytes
 % reconsulting library(error)...
 % reconsulted /usr/local/share/Yap/error.yap in module prolog_xref, 1 msec 256 bytes
reconsulted /usr/local/share/Yap/prolog_xref.pl in module prolog_xref, 96 msec 875568 bytes
true
?- module_property(lists, file(File)).
File = '/usr/local/share/Yap/lists.yap' ? 

yes
?- module_property(lists, file(File)), xref_source(File).
/usr/local/share/Yap/operators.pl:105:0 error while executing prolog_operator:push_operators/1:
          ** unbound variable **

          error raised by foreign-function CallError(), at /Users/pmoura/Documents/Prolog/yap-6.3/C/exec.c:213:0: 
          user provided data is: 'call/1'

[vscosta]

This is a bit more complicated, I'm working on i t