Two stars in a Prolog list

Question

what are the two stars in a list?

[53, 5, 1, 53, 97, 115, 53, 50, 52, 121, 55, 56, 55, 97, 4, 1, 98, **]

I tried searching but no success.

Answer 1

The stars indicate that the term contains itself, e.g.

?- X = f(X).
X = f(**).

?- L = [53, L].
L = [53, **].

This is the case at least in older versions of SWI-Prolog. See also https://lists.iai.uni-bonn.de/pipermail/swi-prolog/2009/001707.html.