Unexpected failure with `lex_chain/1`

```
?- lex_chain([[1],[2,3]]).
   true. % Different sizes accepted.
?- lex_chain([[1],[1,2]]).
   false. % Unexpected
```
Expected: `true`.

Also:
```
?- lex_chain([[],[]]).
   true.
?- lex_chain([[],[1]]).
   false. % Unexpected.
```