This example is chosen to demonstrate that having information loss in both forward and backward directions can be quite natural.
This can be viewed as a motivation for symmetric approaches (e.g., symmetric edit lenses or also TGGs, constraint-based approaches, etc.).
The data structures involved are (1) a tree structure representing a file system with folders and files, and (2) a list representing entries displayed in a web (cat) picture gallery.
A concrete example is depicted in the image below.
Note that the entries have tags that are not present in the file system, and that the folder structure of the file system cannot be induced from the list on the website.
A pair of file system and web gallery models is consistent if there is a bijection between all files in the file system and all entries in the web picture gallery.
A series of possible changes and expected propagation is discussed in these slides.
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Martin Hofmann, Anthony Anjorin