From ea356a539aa2bff831b9a64f1ba0913f63840d19 Mon Sep 17 00:00:00 2001
From: Simon Forman <sforman@hushmail.com>
Date: Tue, 5 May 2020 15:52:40 -0700
Subject: [PATCH] `primrec` combinator in Prolog

Ticket #40375
---
 thun/thun.pl | 9 +++++++++
 1 file changed, 9 insertions(+)

diff --git a/thun/thun.pl b/thun/thun.pl
index aa6190a..f4699f7 100644
--- a/thun/thun.pl
+++ b/thun/thun.pl
@@ -372,6 +372,15 @@ combo(genrec, [R1, R0, Then, If|S],
               [  Else, Then, If|S], E, [ifte|E]) :-
     append(R0, [[If, Then, R0, R1, symbol(genrec)]|R1], Else).
 
+combo(primrec, [list(_), list(B), int(N)|S], S, E, Eo) :-
+    N #=< 0,
+    append(B, E, Eo).
+combo(primrec, [list(R), list(B), int(N)        |S],
+               [list(R), list(B), int(M), int(N)|S], E, Eo) :-
+    N #> 0,
+    M #= N - 1,
+    append([symbol(primrec)|R], E, Eo).
+
 /*
 This is a crude but servicable implementation of the map combinator.