gistfile1.txt

Left recursion:
This arises when a rules causes the invocation of a goal that is essentially equivalent
to the original goal that caused the rule to be used.

This will recurse infinitely
person(adam).
person(X):-person(Y), mother(X,Y).
?-person(X).

This works (according to the book, either fails or something, but doesn't recurse infinitely)
person(X):-person(Y), mother(X,Y).
person(adam).
?-person(X).