The papers describing lean and it's core operations are deliberately small. The kernel of a theorem prover is meant to be such that a human can check it for correctness. The kernel does not include things like type inference or execution. It literally checks the proof and that's all and everything needs to match exactly. It's fairly trivial to write such a thing.
Humans have to establish trust in this.
After that, the rest of the language is written. Yes it is possible the type inference engine is broken but in that case the kernel will reject it
Humans have to establish trust in this.
After that, the rest of the language is written. Yes it is possible the type inference engine is broken but in that case the kernel will reject it