What is the point of Turing-complete blockchains?
In reality, you only want to send cryptocoins from A to B anyway. So what's the point of Ethereum, for example, being Turing-complete?
A proof of decidability (computability and complexity)?
I would like to know how to show this statement "If X is semi-decidable and Y is decidable, then X \ Y is semi-decidable". Can someone please prove this who is good at decidability?