Something anomaly is there around maplist, or is this normal ? It seems that `_S1`

and `_S5`

are different.

```
% ?- X = [_S1, _S4, _S5],
% _S1 = s(_S1, s(0, _S2)),
% _S2 = s(_S2, _S2),
% _S3 = s(_S3, 1),
% _S4 = s(_S3, _S4),
% _S5 = s(s(_S5, 0), _S5),
% maplist(writeln, X).
%@ @(S_1,[S_1=s(S_1,s(0,S_2)),S_2=s(S_2,S_2)])
%@ @(S_2,[S_1=s(S_1,1),S_2=s(S_1,S_2)])
%@ @(S_1,[S_1=s(s(S_1,0),S_1)])
%@ X = [_S1, _S4, _S5], % where
%@ _S1 = s(_S1, s(0, _S2)),
%@ _S2 = s(_S2, _S2),
%@ _S3 = s(_S3, 1),
%@ _S4 = s(_S3, _S4),
%@ _S5 = s(s(_S5, 0), _S5),
%@ _S1 = s(_S1, s(0, _S2)),
%@ _S4 = s(_S3, _S4),
%@ _S5 = s(s(_S5, 0), _S5),
%@ _S2 = s(_S2, _S2),
%@ _S3 = s(_S3, 1).
```

```
% ?- X = [_S1, _S4, _S5],
% _S1 = s(_S1, s(0, _S2)),
% _S2 = s(_S2, _S2),
% _S3 = s(_S3, 1),
% _S4 = s(_S3, _S4),
% _S5 = s(s(_S5, 0), _S5),
% _S1 == _S5.
%@ false.
```