Thanks for the kind feedback. It's a young project to be honest, but I'm pretty proud of what we've done with only two contributors so far. With community participation I think we could support many more languages pretty quickly!
#Cell 1
a = [1,2,3]
b = (a,True)
#Cell 2
b[0][0] = 5
#Cell 3
print(sum(a))
# Cell 2'
b[0][0] = 4
Currently it takes a shallow copy of the state output by each cell, meaning every value is going to be a primitive value or a reference. If it's a reference to mutable state, you're kind of on your own with respect to keeping reproducibility. I felt like this was a good compromise between strictly enforced reproducibility and practicality; if it turns out to be confusing we could consider deep copying the state, or having an option to do that (I could imagine it being pretty bad for efficiency in a lot of ML use cases, though).