Hi, I know that this is probably not the correct place to ask as my question is not directly related to GHC itself, but I thought here are the people that can most likely answer this. To better understand PL papers, especially those involving System Fc and its extensions, I started to write a formal proof of type safety including alpha-equivalence. Currently I have a complete proof for System F https://github.com/jvanbruegge/isabelle-lambda-calculus (without coercions and data types yet). I mainly used the System Fc paper https://www.microsoft.com/en-us/research/wp-content/uploads/2007/01/tldi22-s... as reference, ignoring all the parts about coercions. 1. In the Fc paper, the rule `AppT` has a judgement delta as assumption, which does not exist. I assumed the `ty` judgement was meant there by looking at the arguments. Is this correct? 2. In the `Abs` rule, the type of the variable is required to be valid and of kind star by the judgement `ty`. In the `Let` rule, this assumption is missing. I tried it like that, but was not able to complete the proofs. Is such an assumption missing there or should I be able to proof it without? 3. In the Fc paper, the types and kinds of datatype declarations are added to the context with a separate judgement that interprets the datatype declarations. In the System Fc pro paper https://www.microsoft.com/en-us/research/wp-content/uploads/2016/02/p53-yorg... (from what I can tell) those types and kinds are assumed to be already part of the initial context. At the moment I prove progress against the empty context, so I guess I would have to relax that to an initial context that only contains types and kinds of type constants and nothing else. Is that correct? What is here the "best practice" in terms of PL research? Thank you all Jan