Example

?- comma_list((a,b,c,d),X).
X = [a, b, c, d].

?- comma_list((a,b,(c,d)),X).
X = [a, b, c, d].

?- comma_list(((a,b),(c,d)),X).
X = [a, b, c, d].

?- comma_list(((a,b),c,d),X).
X = [a, b, c, d].

?- comma_list((((a,b),c),d),X).  % associated left on input
X = [a, b, c, d].

?- comma_list(XL,[a,b,c,d]),write_canonical(XL).  % associated right on output
','(a,','(b,','(c,d)))
XL =  (a, b, c, d).