Top new questions this week:
 | 
| 
 This is somewhat conceptual beginner’s question about proof assistants.  I’ve been re-reading the famous Seven Trees in One / Objects of categories as complex numbers. The gist: The type $T$ of binary … 
 | 
| 
 This is a 3rd in a series of questions, spurred by my attempts to encode an argument by Danielsson [1] [2] regarding existence of syntactically non-strictly positive datatype.  The idea (rephrased):   … 
 | 
| 
 So, obviously for a term $t$ of type $T$, I would represent it as:        T  +———–+  |           |  |     t     |  |           |  +———–+    That is a node … 
 | 
| 
 By HOL I mean something like inference rules of HOL Light with the 3 axioms of infinity, extensionality and choice ($\varepsilon$ operator).  By predicative dependent types, I am thinking of MLTT + W-… 
 | 
| 
 I’m wondering how much overlap (read: code-reusage) there is between a PA and an ATP system.  Are they based upon the same type system at least?  I’m wondering, because right now I’m working on … 
 | 
Greatest hits from previous weeks:
 | 
| 
 I keep seeing these phrasing in some proof assistants/elaborators and their issues/internal discussions (e.g. Github search results in cooltt), that seems not that related to the actual proofs/… 
 | 
| 
 I’ve started to play with mechanizing some set theory stuff. I’m not sure if I want a constructive flavor or not yet.  Anyhow you can do stuff like axiomize the empty set  $$ \top \vdash \exists P. \… 
 | 
| 
 Coq’s built-in termination checker accepts some rather intricate recursion patterns with functional values in data types, as shown by this example  … 
 | 
| 
 For a project I’m planning on doing with some students this summer, I’m looking for an implementation of normalization-by-evaluation whose code the students can read, understand, and extend.  So I’d … 
 | 
| 
 What is a good starting point to learn about proof assistants?  The answer will invariably depend on the area of interest: mathematics (and its areas, e.g. algebra,combinatorics, analysis, logic), CS – … 
 | 
| 
 Type systems, and the proof assistants based on them, are frequently divided into predicative and impredicative.  What exactly does this mean? I’ve heard the slogan “impredicativity means you can’… 
 | 
| 
 A type that is a member of a universe can be coerced into a higher universe. Is that coercion injective? That is, if two elements of U1 are equal after being coerced to U2, does that imply they are … 
 |