what are the two stars on the list?
[53, 5, 1, 53, 97, 115, 53, 50, 52, 121, 55, 56, 55, 97, 4, 1, 98, **]
I tried the search but did not succeed.
Stars indicate that the term contains itself, for example
?- X = f(X). X = f(**). ?- L = [53, L]. L = [53, **].
This is at least in earlier versions of SWI-Prolog. See also https://lists.iai.uni-bonn.de/pipermail/swi-prolog/2009/001707.html .