[deadbeef57]

Ok, no worries. I understand that from a theoretical point it's not so nice that defeq is not decidable. But frankly I don't care. Because in practice, I don't know of any example where someone was bitten by this. It only shows up in contrived examples. And even if you have some practical example where lean gets stuck on A = B, it will probably be very easy to find a C so that lean gets unstuck on A = C = B.

[Edit, this comment only replies to the first paragraph of parent. I wrote it before parent was edited.]

[xvilka]

By the way, just for the record, I like many aspects of the new Lean 4 design and wish the project luck. They really aim for better maintainability and user experience. It is nice to see them taking best solutions from other programming languages and frameworks.