ERROR: asserta/1: No permission to modify static_procedure `$shell/2'

I built a x86_64-linux SWI-Prolog.  I would like to undefined (or redefine)
'$shell'/2.

I am not sure what I am missing or was it just the 64bit build? 




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?- redefine_system_predicate('$shell'/2).

Yes
?- asserta('$shell'(_,_)).
ERROR: asserta/1: No permission to modify static_procedure `$shell/2'


?- predicate_property('$shell'(_,_),X).

X = built_in ;

X = foreign ;

X = imported_from(system) ;

X = notrace ;

X = nodebug ;

No


What am I forgetting?

Thank you in advance.