Unexpected loop with goal expansion

[notoria]

I tried this code:

?- [user].
:- use_module(library(format), [portray_clause/2]).
:- use_module(library(lists), [append/3]).

is_convertible_term(T) :-
    catch((! ; T), error(type_error(callable,_),_), false).

is_body(T) :-
    \+ \+ (
        term_variables(T, Vs),
        append(Vs, _, [0|Vs]),
        is_convertible_term(T)
    ).

goal_expansion(G, _) :-
    portray_clause(user_error, goal-G),
    false.
goal_expansion(G, call(G)) :-
    \+ is_body(G).

head :-
    call((a, b)),
    call(','(c), d),
    call(',', e, f).

The expansion loops because invalid bodies are introduced and Scryer expands dynamic goals at compile-time:

goal-(call((a,b)),call(','(c),d),call(',',e,f)).
goal-call((a,b)).
goal-(a,b).
goal-a.
goal-b.
goal-(call(','(c),d),call(',',e,f)).
goal-call(','(c),d).
goal-(c,A).
goal-c.
goal-call(',',e,f).
goal-(A,B).

Maybe goal_expansion(G, call(G)) :- \+ is_body(G) is expected to loop since dynamic goals are expanded but the body of head/0 is valid, it should not loop here.

[notoria]

Unexpected:

?- [user].
goal_expansion(a(A,B), b) :-
    var(B).

:- dynamic(test/0).
test :-
    call(a(_, 0)),
    call(a(_), 1),
    call(a, _, 2).
% Warning: singleton variables A at line 1 of user

?- clause(test, G).
   G = (call(user:a(_A,0)),call(user:b,1),call(user:b,_B,2)). % unexpected.
?-

Expected: G = (call(user:a(_A,0)),call(user:a,1),call(user:a,_B,2)).