From b15d2bc3ddd66abac69ef0a159b010743544b5ee Mon Sep 17 00:00:00 2001
From: Mohamed Barakat <mohamed.barakat@rwth-aachen.de>
Date: Tue, 27 Jun 2023 17:39:45 +0200
Subject: [PATCH] added error message to constructors
 MeetSemilatticeOfSingle/MultipleDifferences

---
 Locales/PackageInfo.g              | 2 +-
 Locales/gap/MultipleDifferences.gi | 5 +++++
 Locales/gap/SingleDifferences.gi   | 6 ++++++
 3 files changed, 12 insertions(+), 1 deletion(-)

diff --git a/Locales/PackageInfo.g b/Locales/PackageInfo.g
index 948e8a746..19426b040 100644
--- a/Locales/PackageInfo.g
+++ b/Locales/PackageInfo.g
@@ -10,7 +10,7 @@ SetPackageInfo( rec(
 
 PackageName := "Locales",
 Subtitle := "Locales, frames, coframes, meet semi-lattices of locally closed subsets, and Boolean algebras of constructible sets",
-Version := "2023.06-13",
+Version := "2023.06-14",
 Date := ~.Version{[ 1 .. 10 ]},
 Date := (function ( ) if IsBound( GAPInfo.SystemEnvironment.GAP_PKG_RELEASE_DATE ) then return GAPInfo.SystemEnvironment.GAP_PKG_RELEASE_DATE; else return Concatenation( ~.Version{[ 1 .. 4 ]}, "-", ~.Version{[ 6, 7 ]}, "-01" ); fi; end)( ),
 License := "GPL-2.0-or-later",
diff --git a/Locales/gap/MultipleDifferences.gi b/Locales/gap/MultipleDifferences.gi
index 779cf06f9..bb5dd825f 100644
--- a/Locales/gap/MultipleDifferences.gi
+++ b/Locales/gap/MultipleDifferences.gi
@@ -31,6 +31,11 @@ InstallMethod( MeetSemilatticeOfMultipleDifferences,
   function( P )
     local name, D;
     
+    if not ( HasIsDistributiveBicartesianProset( P ) and IsDistributiveBicartesianProset( P ) ) then
+        Error( "the input category `P` must be a distributive (bicartesian) proset (e.g. a distributive lattice) ",
+               "for the meet-semilattice of multiple differences to be well-defined\n" );
+    fi;
+    
     name := "The meet-semilattice of multiple differences of ";
     
     name := Concatenation( name, Name( P ) );
diff --git a/Locales/gap/SingleDifferences.gi b/Locales/gap/SingleDifferences.gi
index 5d394ebd0..9050c7d7c 100644
--- a/Locales/gap/SingleDifferences.gi
+++ b/Locales/gap/SingleDifferences.gi
@@ -36,6 +36,11 @@ InstallMethod( MeetSemilatticeOfSingleDifferences,
   function( P )
     local name, D, L;
     
+    if not ( HasIsDistributiveBicartesianProset( P ) and IsDistributiveBicartesianProset( P ) ) then
+        Error( "the input category `P` must be a distributive (bicartesian) proset (e.g. a distributive lattice) ",
+               "for the meet-semilattice of single differences to be well-defined\n" );
+    fi;
+    
     name := "The meet-semilattice of single differences of ";
     
     name := Concatenation( name, Name( P ) );
@@ -84,6 +89,7 @@ InstallMethod( MeetSemilatticeOfSingleDifferences,
         
     end );
     
+    ## the following is independent of the representatives of the single differences when the given bicartesian proset is (co)distributive
     ## (A - A') ≤_D (B - B') ⟺  ( A ≤_C ( A' ∨ B ) ) and ( ( A ∧ B' ) ≤_C A' )
     AddIsHomSetInhabited( D,
       function( D, A, B )