diff --git a/src/group_cmd.cpp b/src/group_cmd.cpp
index be1bfce..5e123d2 100644
--- a/src/group_cmd.cpp
+++ b/src/group_cmd.cpp
@@ -314,7 +314,7 @@ CommandCost CmdDeleteGroup(TileIndex tile, DoCommandFlag flags, uint32 p1, uint3
 	/* Remove all vehicles from the group */
 	DoCommand(0, p1, 0, flags, CMD_REMOVE_ALL_VEHICLES_GROUP);

-	/* Delete sub-groups */
+	/* Delete subgroups. */
 	Group *gp;
 	FOR_ALL_GROUPS(gp) {
 		if (gp->parent == g->index) {
@@ -402,11 +402,7 @@ CommandCost CmdAlterGroup(TileIndex tile, DoCommandFlag flags, uint32 p1, uint32

 			/* Ensure request parent isn't child of group.
 			 * This is the only place that infinite loops are prevented. */
-			const Group *looptest = pg;
-			while (looptest->parent != INVALID_GROUP) {
-				if (looptest->parent == g->index) return CMD_ERROR;
-				looptest = Group::Get(looptest->parent);
-			}
+			if (GroupIsInGroup(pg->index, g->index)) return CMD_ERROR;
 		}

 		if (flags & DC_EXEC) {
@@ -579,7 +575,7 @@ CommandCost CmdRemoveAllVehiclesGroup(TileIndex tile, DoCommandFlag flags, uint3
 }

 /**
- * Set replace protection for a group and its sub-groups.
+ * Set replace protection for a group and its subgroups.
  * @param g initial group.
  * @param protect 1 to set or 0 to clear protection.
  */
@@ -601,7 +597,7 @@ static void SetGroupReplaceProtection(Group *g, bool protect)
  * - p1 bit 0-15 : GroupID
  * @param p2
  * - p2 bit 0    : 1 to set or 0 to clear protection.
- * - p2 bit 1    : 1 to apply to sub-groups.
+ * - p2 bit 1    : 1 to apply to subgroups.
  * @param text unused
  * @return the cost of this operation or an error
  */
@@ -686,7 +682,7 @@ void UpdateTrainGroupID(Train *v)

 /**
  * Get the number of engines with EngineID id_e in the group with GroupID
- * id_g
+ * id_g and its subgroups.
  * @param company The company the group belongs to
  * @param id_g The GroupID of the group used
  * @param id_e The EngineID of the engine to count
@@ -721,10 +717,7 @@ void RemoveAllGroupsForCompany(const CompanyID company)
  */
 bool GroupIsInGroup(GroupID search, GroupID group)
 {
-	if (search == NEW_GROUP ||
-	    search == ALL_GROUP ||
-	    search == DEFAULT_GROUP ||
-	    search == INVALID_GROUP) return search == group;
+	if (!Group::IsValidID(search)) return search == group;

 	do {
 		if (search == group) return true;
diff --git a/src/group_gui.cpp b/src/group_gui.cpp
index 0b6c860..17f7836 100644
--- a/src/group_gui.cpp
+++ b/src/group_gui.cpp
@@ -30,7 +30,7 @@

 #include "table/sprites.h"

-static const int LEVEL_WIDTH = 10; ///< Indenting width of a sub-group in pixels
+static const int LEVEL_WIDTH = 10; ///< Indenting width of a subgroup in pixels.

 typedef GUIList<const Group*> GUIGroupList;

@@ -549,7 +549,7 @@ public:

 			case WID_GL_LIST_VEHICLE:
 				if (this->vli.index != ALL_GROUP) {
-					/* Mark vehicles which are in sub-groups */
+					/* Mark vehicles which are in subgroups. */
 					int y = r.top;
 					uint max = min(this->vscroll->GetPosition() + this->vscroll->GetCapacity(), this->vehicles.Length());
 					for (uint i = this->vscroll->GetPosition(); i < max; ++i) {