An extremely common pattern in virtually all "programmers' text editors" is that of management of indentation level. Our simplified example case here assumes tab-based indentation, with an "obvious error" present in the sample text to correct using this mechanism.
This technique is far and away the most complex of the experiments in this set so far. To try to keep things within the realm of sensibility, assumptions have been made: all indentation is tab-based.
Support for other forms of indentation (preferably configurable!) left as an exersize for the reader.