From 6de82436d3ab5f9288a527875666c8e1a1e60580 Mon Sep 17 00:00:00 2001
From: zljlzljlz <zlj1@st-andrews.ac.uk>
Date: Wed, 9 Oct 2024 15:59:22 +0100
Subject: [PATCH] Added IsOrderFilter

---
 gap/oper.gd | 1 +
 gap/oper.gi | 5 +++++
 2 files changed, 6 insertions(+)

diff --git a/gap/oper.gd b/gap/oper.gd
index e3c14d3dd..5c9f6878a 100644
--- a/gap/oper.gd
+++ b/gap/oper.gd
@@ -142,6 +142,7 @@ DeclareOperation("DigraphShortestPathSpanningTree", [IsDigraph, IsPosInt]);
 DeclareOperation("VerticesReachableFrom", [IsDigraph, IsPosInt]);
 DeclareOperation("VerticesReachableFrom", [IsDigraph, IsList]);
 DeclareOperation("IsOrderIdeal", [IsDigraph, IsList]);
+DeclareOperation("IsOrderFilter", [IsDigraph, IsList]);
 DeclareOperation("Dominators", [IsDigraph, IsPosInt]);
 DeclareOperation("DominatorTree", [IsDigraph, IsPosInt]);
 DeclareOperation("DigraphCycleBasis", [IsDigraph]);
diff --git a/gap/oper.gi b/gap/oper.gi
index 5703ec7a5..b22411d3a 100644
--- a/gap/oper.gi
+++ b/gap/oper.gi
@@ -2312,6 +2312,11 @@ function(D, roots)
 
 end);
 
+InstallMethod(IsOrderFilter, "for a digraph and a list of vertices", [IsDigraph, IsList],
+function(D, roots)
+  return IsOrderIdeal(DigraphReverse(D), roots);
+end);
+
 InstallMethod(DominatorTree, "for a digraph and a vertex",
 [IsDigraph, IsPosInt],
 function(D, root)