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A pure python demonstration of the problem looks like this (`__file__` stores the path to the executed module):
```python
with open(__file__) as f:
 src = f.read()
with open(__file__, 'w') as f:
 f.write('\n\n\n\n\n# Whoops! Wrong line!\n')
 f.write(src)
raise NotImplementedError('The prepended lines have confused you.')
```