From f2b63d1689c58be1c8b644d902d4e698aec653e6 Mon Sep 17 00:00:00 2001
From: Markus Triska <triska@metalevel.at>
Date: Fri, 5 Jan 2024 18:50:08 +0100
Subject: [PATCH] Prevent assertion of clauses for (-->)/2 when DCGs are
 enabled.

This addresses #1508.
---
 src/lib/dcgs.pl | 10 +++++++++-
 1 file changed, 9 insertions(+), 1 deletion(-)

diff --git a/src/lib/dcgs.pl b/src/lib/dcgs.pl
index 550009bb8..ef2e82533 100644
--- a/src/lib/dcgs.pl
+++ b/src/lib/dcgs.pl
@@ -14,7 +14,8 @@
            phrase/3,
            seq//1,
            seqq//1,
-           ... //0
+           ... //0,
+           (-->)/2
           ]).
 
 :- use_module(library(error)).
@@ -220,3 +221,10 @@
     ).
 
 user:goal_expansion(phrase(GRBody, S), phrase(GRBody, S, [])).
+
+
+% (-->)/2 behaves as if it didn't exist. We export (and define) it
+% only so that clauses for (-->)/2 cannot be asserted when
+% library(dcgs) is loaded.
+
+(_-->_) :- throw(error(existence_error(procedure,(-->)/2),(-->)/2)).