Programmers should concentrate on invariants in the software.
And it was interesting to see them apply this is in a very specific case (where I happened to understand the problem and the context).
And I wondered whether programming languages and libraries could support this approach better.
This paper and talk by Bob Atkey looks like it is very relevant: From Parametricity to Conservation Laws, via Noether's Theorem (slides). Unfortunately understanding it is a bit of a way down my stack.