3 files changed, 8 insertions, 8 deletions
diff --git a/courses.go b/courses.go
index c92a3d2..ccb5313 100644
--- a/courses.go
+++ b/courses.go
@@ -193,7 +193,7 @@ func setupCourses() error {
 	return nil
 }
 
-type userCourseGroupsT map[courseGroupT]bool
+type userCourseGroupsT map[courseGroupT]struct{}
 
 func populateUserCourseGroups(
 	ctx context.Context,
@@ -239,7 +239,7 @@ func populateUserCourseGroups(
 			defer coursesLock.RUnlock()
 			thisGroupName = courses[thisCourseID].Group
 		}()
-		if (*userCourseGroups)[thisGroupName] {
+		if _, ok := (*userCourseGroups)[thisGroupName]; ok {
 			return fmt.Errorf(
 				"%w: user %v, group %v",
 				errMultipleChoicesInOneGroup,
@@ -247,7 +247,7 @@ func populateUserCourseGroups(
 				thisGroupName,
 			)
 		}
-		(*userCourseGroups)[thisGroupName] = true
+		(*userCourseGroups)[thisGroupName] = struct{}{}
 	}
 	return nil
 }
diff --git a/wsc.go b/wsc.go
index 6da50da..4142512 100644
--- a/wsc.go
+++ b/wsc.go
@@ -142,7 +142,7 @@ func handleConn(
 	 * userCourseGroups stores whether the user has already chosen a course
 	 * in the courseGroup.
 	 */
-	var userCourseGroups userCourseGroupsT = make(map[courseGroupT]bool)
+	var userCourseGroups userCourseGroupsT = make(map[courseGroupT]struct{})
 	err := populateUserCourseGroups(newCtx, &userCourseGroups, userID)
 	if err != nil {
 		return reportError(
diff --git a/wsm.go b/wsm.go
index 10e0889..c163868 100644
--- a/wsm.go
+++ b/wsm.go
@@ -112,7 +112,7 @@ func messageChooseCourse(
 
 	thisCourseGroup := course.Group
 
-	if (*userCourseGroups)[thisCourseGroup] {
+	if _, ok := (*userCourseGroups)[thisCourseGroup]; ok {
 		err := writeText(ctx, c, "R "+mar[1]+" :Group conflict")
 		if err != nil {
 			return fmt.Errorf(
@@ -201,7 +201,7 @@ func messageChooseCourse(
 			 * This would race if message handlers could run
 			 * concurrently for one connection.
 			 */
-			(*userCourseGroups)[thisCourseGroup] = true
+			(*userCourseGroups)[thisCourseGroup] = struct{}{}
 
 			err = writeText(ctx, c, "Y "+mar[1])
 			if err != nil {
@@ -308,10 +308,10 @@ func messageUnchooseCourse(
 			defer coursesLock.RUnlock()
 			thisCourseGroup = courses[courseID].Group
 		}()
-		if !(*userCourseGroups)[thisCourseGroup] {
+		if _, ok := (*userCourseGroups)[thisCourseGroup]; !ok {
 			return reportError("inconsistent user course groups")
 		}
-		(*userCourseGroups)[thisCourseGroup] = false
+		delete(*userCourseGroups, thisCourseGroup)
 	}
 
 	err = writeText(ctx, c, "N "+mar[1])