(base) 538$ swipl
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?- [anbn2].
true.

?- a(Tree, [a,a,a,b,b,b], []).
Tree = a(a, b(a(a, b(a(a, b(b)), b)), b)) ;
false.

?- a(Tree, [a,a,b,b], []).
Tree = a(a, b(a(a, b(b)), b)) ;
false.

?- a(Tree, [a,a,b,b,b], []).
false.

?- [lrrg].
true.

?- s(Sheep,[]).
Sheep = [b, a, a, !] ;
Sheep = [b, a, a, a, !] ;
Sheep = [b, a, a, a, a, !] ;
Sheep = [b, a, a, a, a, a, !] ;
Sheep = [b, a, a, a, a, a, a, !] ;
Sheep = [b, a, a, a, a, a, a, a, !] ;
Sheep = [b, a, a, a, a, a, a, a, a|...] ;
Sheep = [b, a, a, a, a, a, a, a, a|...] ;
Sheep = [b, a, a, a, a, a, a, a, a|...] ;
Sheep = [b, a, a, a, a, a, a, a, a|...] ;
Sheep = [b, a, a, a, a, a, a, a, a|...] .

?- [lrrg2].
Warning: /Users/sandiway/Desktop/538/lrrg2.prolog:1:
Warning:    Redefined static procedure s/2
Warning:    Previously defined at /Users/sandiway/Desktop/538/lrrg.prolog:1
Warning: /Users/sandiway/Desktop/538/lrrg2.prolog:2:
Warning:    Redefined static procedure a/2
Warning:    Previously defined at /Users/sandiway/Desktop/538/lrrg.prolog:2
Warning: /Users/sandiway/Desktop/538/lrrg2.prolog:4:
Warning:    Redefined static procedure ba/2
Warning:    Previously defined at /Users/sandiway/Desktop/538/lrrg.prolog:4
Warning: /Users/sandiway/Desktop/538/lrrg2.prolog:5:
Warning:    Redefined static procedure b/2
Warning:    Previously defined at /Users/sandiway/Desktop/538/lrrg.prolog:5
true.

?- [lrrg2].
true.

?- s(Sheep,[]).
ERROR: Stack limit (1.0Gb) exceeded
ERROR:   Stack sizes: local: 1.0Gb, global: 48.8Mb, trail: 0Kb
ERROR:   Stack depth: 6,390,835, last-call: 0%, Choice points: 6,390,826
ERROR:   Probable infinite recursion (cycle):
ERROR:     [6,390,835] user:a(_12787434, _12787436)
ERROR:     [6,390,834] user:a(_12787454, _12787456)
   Exception: (6,386,195) a(_5396, _12778088) ? creep
   Exception: (6,386,194) a(_5396, _12778086) ? abort
% Execution Aborted
?-