diff --git a/doc/environments/groupthm/groupthm.dtx b/doc/environments/groupthm/groupthm.dtx
index e68acd1..6e71bfb 100644
--- a/doc/environments/groupthm/groupthm.dtx
+++ b/doc/environments/groupthm/groupthm.dtx
@@ -63,14 +63,9 @@
 % \newpage
 % 
 % \begin{itemize}
-%   \item Inheritance
-%   \item Appending to groups
-%   \item default group integration
-%   \item better error message to avoid low level tex errors in any case
+%   \item appending to groups
 %   \item potentially: detection of star / nonstar misuse
 %   \item add command to define group rule for all existing groups
-%   \item various info commands on defined groups?
-%   \item debugging information?
 % \end{itemize}
 % A central thing in \LaTeX is the usage of \enquote{theorems}.
 % With \enquote{theorems} we actually mean \enquote{environments} that typically
@@ -1169,6 +1164,14 @@
 \bool_new:N  \l_@@_in_family_options_environment_bool
 %    \end{macrocode}
 % \end{variable}
+% \begin{variable}{\g_@@_append_groups_int}
+%		 
+%    This counts the number of times we appended to a group. 
+%
+%    \begin{macrocode}
+\int_new:N   \g_@@_append_groups_int
+%    \end{macrocode}
+% \end{variable}
 % 
 %
 % \subsection{Key interface}
@@ -1450,9 +1453,15 @@
 %    \end{macrocode}
 % \begin{variable}{ \g_@@_parents_group__\meta{theorem group}__clist }
 %     
-% This variable will accumulate the parents of this group:
+% This variable will accumulate the parents of this group.
 %    \begin{macrocode}
     \clist_new:c { g_@@_parents_group__#1__clist }
+%    \begin{macrocode}
+% This ensures default priorities between groups.
+%    \end{macrocode}
+    \hook_gset_rule:nnnn { ?? } { all        } { before } { #1 }
+    \hook_gset_rule:nnnn { ?? } { unnumbered } { after  } { #1 }
+    \hook_gset_rule:nnnn { ?? } { starred    } { after  } { #1 }
 %    \end{macrocode}
 % \end{variable}
 % Add defined group to corresponding list 
@@ -1782,6 +1791,7 @@
       \l_@@_thmtools_clist
   }
 %    \end{macrocode}
+\cs_generate_variant:Nn \groupthm_new_group:nn { n x }
 % \end{macro}
 %
 %
@@ -1850,20 +1860,21 @@
     \str_set:Nx \l_tmpa_str { \tl_trim_spaces:n { #3 } }
     \str_if_eq:VnT \l_tmpa_str { higher }
       {
-        \str_set:Nn \l_tmpa_tl { after }
+        \str_set:Nn \l_tmpa_str { after }
       }
     \str_if_eq:VnT \l_tmpa_str { lower }
       {
-        \str_set:Nn \l_tmpa_tl { before }
+        \str_set:Nn \l_tmpa_str { before }
       }
     \str_if_eq:nnTF { #1 } { ?? }
       {
-        \hook_gset_rule:nnVn {??} {#2} \l_tmpa_tl {#4}
+        \hook_gset_rule:nnVn {??} {#2} \l_tmpa_str {#4}
       }
       {
-        \hook_gset_rule:nnVn { @@ / #1 } {#2} \l_tmpa_tl {#4}
+        \hook_gset_rule:nnVn { @@ / #1 } {#2} \l_tmpa_str {#4}
       }
   }
+\cs_generate_variant:Nn \groupthm_declare_group_rule:nnnn { n n n x }
 %    \end{macrocode}
 % \end{macro}
 
@@ -1897,6 +1908,7 @@
     \@@_ensure_group_exists:n { #2 }
     \clist_gput_left:cn { g_@@_parents_group__#1__clist } { #2 }
   }
+\cs_generate_variant:Nn \groupthm_add_parent:nn { n x }
 %    \end{macrocode}
 % \end{macro}
 %
@@ -1965,6 +1977,51 @@
 %    \end{macrocode}
 % \end{macro}
 % 
+% \begin{macro}{\groupthm_append_to_group:nn}
+%    \begin{syntax}
+%       \cs{groupthm_append_to_group:nn}\marg{key=value list}\marg{theorem group}
+%    \end{syntax}
+%		 
+%    This works the same as defining a new group, except that we append to the group,
+%    overwriting the old group in case of conflicts.
+%
+%    \begin{macrocode}
+\cs_new:Npn \groupthm_append_to_group:nn #1 #2
+  {
+    \groupthm_new_group:nx  { #1 } { __append__ \int_use:N \g_@@_append_groups_int }
+    \groupthm_add_parent:nx { #2 } { __append__ \int_use:N \g_@@_append_groups_int }
+    \groupthm_declare_group_rule:nnnx
+      { ?? } { #2 } { before } { __append__ \int_use:N \g_@@_append_groups_int }
+    \int_gincr:N \g_@@_append_groups_int
+  }
+%    \end{macrocodel}
+% \end{macro}
+% 
+
+
+% \begin{macro}{\AppendToTheoremGroup}
+%    \begin{syntax}
+%       \cs{AppendToTheoremGroup} \marg{key=value list}\marg{theorem group}
+%    \end{syntax}
+%		 
+%     
+%
+%    \begin{macrocode}
+\NewDocumentCommand { \AppendToTheoremGroup } { O{} m }
+  {
+    \groupthm_append_to_group:nn { #1 } { #2 }
+  }
+%    \end{macrocode}
+% \end{macro}
+
+
+%   We also provide the three default groups:
+% 
+%    \begin{macrocode}
+\groupthm_new_group:nnnnn { all        } { } { } { } {               }
+\groupthm_new_group:nnnnn { starred    } { } { } { } { numbered = no }
+\groupthm_new_group:nnnnn { unnumbered } { } { } { } { numbered = no }
+%    \end{macrocode}
 % 
 % \subsection{Iterating over powersets}
 % For generating the different variants of a theorem family,
@@ -2120,7 +2177,7 @@
 %    \end{macrocode}
 % Now, retrieve the group properties, by writing these into the hooks
 %    \begin{macrocode}
-    \@@_flatten_groups_hierarchy:nN { #2 } \l_@@_group_clist
+    \@@_flatten_groups_hierarchy:nN { #2, all } \l_@@_group_clist
     \clist_map_function:NN \l_@@_group_clist \@@_use_group:n
     \tl_log:x { Flattened ~ groups ~ '#2' ~ to ~ '\clist_use:Nn \l_@@_group_clist {,}' ~ when ~
                 defining ~ theorem ~ '#1' }
diff --git a/doc/environments/groupthm/groupthm.sty b/doc/environments/groupthm/groupthm.sty
index 2cb91e8..0346662 100644
--- a/doc/environments/groupthm/groupthm.sty
+++ b/doc/environments/groupthm/groupthm.sty
@@ -87,6 +87,7 @@
 \clist_new:N \l__groupthm_group_clist
 \clist_new:N \g__groupthm_defined_groups_clist
 \bool_new:N  \l__groupthm_in_family_options_environment_bool
+\int_new:N   \g__groupthm_append_groups_int
 \keys_define:nn { groupthm / theorem ~ group }
   {
     prefix      .tl_set:N      = \l__groupthm_key_prefix_tl,
@@ -215,6 +216,9 @@
     }
     \__groupthm_add_to_group_ordering:n { #1 }
     \clist_new:c { g__groupthm_parents_group__#1__clist }
+    \hook_gset_rule:nnnn { ?? } { all        } { before } { #1 }
+    \hook_gset_rule:nnnn { ?? } { unnumbered } { after  } { #1 }
+    \hook_gset_rule:nnnn { ?? } { starred    } { after  } { #1 }
     \clist_gput_left:Nn \g__groupthm_defined_groups_clist { #1 }
   }
 \cs_new:Npn \__groupthm_undefine_group:n #1
@@ -361,6 +365,7 @@
       \l__groupthm_mapname_clist
       \l__groupthm_thmtools_clist
   }
+\cs_generate_variant:Nn \groupthm_new_group:nn { n x }
 \cs_new:Npn \__groupthm_new_document_command:Nnn #1 #2 #3
 {
   \NewDocumentCommand { #1 } { #2 } { #3 }
@@ -387,20 +392,21 @@
     \str_set:Nx \l_tmpa_str { \tl_trim_spaces:n { #3 } }
     \str_if_eq:VnT \l_tmpa_str { higher }
       {
-        \str_set:Nn \l_tmpa_tl { after }
+        \str_set:Nn \l_tmpa_str { after }
       }
     \str_if_eq:VnT \l_tmpa_str { lower }
       {
-        \str_set:Nn \l_tmpa_tl { before }
+        \str_set:Nn \l_tmpa_str { before }
       }
     \str_if_eq:nnTF { #1 } { ?? }
       {
-        \hook_gset_rule:nnVn {??} {#2} \l_tmpa_tl {#4}
+        \hook_gset_rule:nnVn {??} {#2} \l_tmpa_str {#4}
       }
       {
-        \hook_gset_rule:nnVn { __groupthm / #1 } {#2} \l_tmpa_tl {#4}
+        \hook_gset_rule:nnVn { __groupthm / #1 } {#2} \l_tmpa_str {#4}
       }
   }
+\cs_generate_variant:Nn \groupthm_declare_group_rule:nnnn { n n n x }
 
 \NewDocumentCommand { \DeclareTheoremGroupRule } { O{??} m m m }
   {
@@ -412,6 +418,7 @@
     \__groupthm_ensure_group_exists:n { #2 }
     \clist_gput_left:cn { g__groupthm_parents_group__#1__clist } { #2 }
   }
+\cs_generate_variant:Nn \groupthm_add_parent:nn { n x }
 \NewDocumentCommand { \AddTheoremGroupParent } { m m }
   {
     \groupthm_add_parent:nn { #1 } { #2 }
@@ -440,6 +447,23 @@
           }
       }
   }
+\cs_new:Npn \groupthm_append_to_group:nn #1 #2
+  {
+    \groupthm_new_group:nx  { #1 } { __append__ \int_use:N \g__groupthm_append_groups_int }
+    \groupthm_add_parent:nx { #2 } { __append__ \int_use:N \g__groupthm_append_groups_int }
+    \groupthm_declare_group_rule:nnnx
+      { ?? } { #2 } { before } { __append__ \int_use:N \g__groupthm_append_groups_int }
+    \int_gincr:N \g__groupthm_append_groups_int
+  }
+
+\NewDocumentCommand { \AppendToTheoremGroup } { O{} m }
+  {
+    \groupthm_append_to_group:nn { #1 } { #2 }
+  }
+
+\groupthm_new_group:nnnnn { all        } { } { } { } {               }
+\groupthm_new_group:nnnnn { starred    } { } { } { } { numbered = no }
+\groupthm_new_group:nnnnn { unnumbered } { } { } { } { numbered = no }
 \clist_new:N  \l__powerset_copied_clist
 \seq_new:N    \l__powerset_saved_seq
 \cs_generate_variant:Nn \clist_remove_all:Nn { N V }
@@ -505,7 +529,7 @@
     \hook_gremove_code:nn { __groupthm/suffix }{*}
     \hook_gremove_code:nn { __groupthm/mapname  }{*}
     \hook_gremove_code:nn { __groupthm/thmtools }{*}
-    \__groupthm_flatten_groups_hierarchy:nN { #2 } \l__groupthm_group_clist
+    \__groupthm_flatten_groups_hierarchy:nN { #2, all } \l__groupthm_group_clist
     \clist_map_function:NN \l__groupthm_group_clist \__groupthm_use_group:n
     \tl_log:x { Flattened ~ groups ~ '#2' ~ to ~ '\clist_use:Nn \l__groupthm_group_clist {,}' ~ when ~
                 defining ~ theorem ~ '#1' }