Immutable data comes to mind as something relatively simple. There is a world of complexity for working with it in a fault-tolerant and efficient way, but from

Immutable data comes to mind as something relatively simple. There is a world of complexity for working with it in a fault-tolerant and efficient way, but from a structural perspective an immutable bit of data is one thing rather than many things and it is only that thing and can only be that thing forever. This strikes at the very core of what simplicity is meant to be, not in the sense of ease, but in the sense of not being complexity.