diff --git a/src/environments/groupthm.pysty3 b/src/environments/groupthm.pysty3
index 7c9b389..e88290d 100644
--- a/src/environments/groupthm.pysty3
+++ b/src/environments/groupthm.pysty3
@@ -54,6 +54,8 @@ __HEADER__(Grouping theorems for easier customization.)
 \tl_new:N    \l_groupthm_mapname_clist
 \tl_new:N    \l_groupthm_thmtools_clist
 
+\tl_new:N    \l_groupthm_relation_tl
+
 
 %%% Keys
 
@@ -160,6 +162,8 @@ __HEADER__(Grouping theorems for easier customization.)
   \hook_gremove_code:nn { groupthm / thmtools }{*}
   
   \clist_map_function:nN { #2 } \group_use:n
+
+%  \ShowHook{ groupthm / postname }
   \hook_use:n { groupthm / prename }
   \hook_use:n { groupthm / postname }
   \hook_use:n { groupthm / mapname }
@@ -211,6 +215,32 @@ __HEADER__(Grouping theorems for easier customization.)
 }
 
 
+\cs_generate_variant:Nn \hook_gset_rule:nnnn { n n V n }
+
+% hook group1 relation group2
+\cs_new:Npn \declare_theorem_group_rule:nnnn #1 #2 #3 #4
+{
+  \tl_set:Nx \l_groupthm_relation_tl { \tl_trim_spaces:n { #3 } }
+
+  \tl_if_eq:nnT { higher } { #3 }
+  {
+    \tl_set:Nn \l_groupthm_relation_tl { after }
+  }
+
+  \tl_if_eq:nnT { lower } { #3 }
+  {
+    \tl_set:Nn \l_groupthm_relation_tl { before }
+  }
+
+  \hook_gset_rule:nnVn {#1} {#2} \l_groupthm_relation_tl {#4}
+}
+
+% hook, group1, relation, group2
+\NewDocumentCommand { \DeclareTheoremGroupRule } { O{??} m m m }
+{
+  \declare_theorem_group_rule:nnnn {#1} {#2} {#3} {#4}
+}
+
 
 %%% Default groups available
 
@@ -237,7 +267,7 @@ __HEADER__(Grouping theorems for easier customization.)
 \NewTheoremGroup
 [
   thmtools = { sibling = insubsection },
-  postname = { * }
+  postname = { * },
 ] { star }
 
 \NewTheoremGroup