is_convertible_term(T) should be true, if T can be converted to a valid body (7.6.2 of ISO/IEC 13211-1).
Thus:
?- is_convertible_term((A,B)).
true.
?- is_convertible_term(A).
true.
?- is_convertible_term(3.14).
false.
?- is_convertible_term((a->b;c,!)).
true.
?- is_convertible_term(\+1).
true. % because (\+)/1 is a built-in predicate
This should be written in ISO Prolog and it should be as short as possible.
Additionally, is_body(T) should be true, if T is identical to the conversion to a valid body. With the help of it we could for example remove call wrappers (provided there is no interference with !/0 or a top level (->)/2).
?- is_body((a,X)).
false.
?- is_body((a,call(X))). % this is the conversion of (a,X)
true.
What I have tried so far, was to start with is_body/1 since it can only be true if is_convertible_term/1 is true. So
is_body(T) :-
is_convertible_term(T),
... . % <-- here I am stuck
(Just for the sake of completeness, we are considering here ISO Prolog only without any constraint extension, but predicates from the Prolog prologue are fine)
Very short solution.
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)
).