Copy on write handles most cases fine. You can also still use such a system as a repl: just add more lines and don't modify previous lines. If you do that then

Copy on write handles most cases fine. You can also still use such a system as a repl: just add more lines and don't modify previous lines. If you do that then nothing needs to be re-executed. Since most people work with small data sets or can use a subset of their dataset during development, I think this would be a better default for a notebook.