diff --git a/doc/part-8-items.adoc b/doc/part-8-items.adoc
index f6c1108..735dc97 100644
--- a/doc/part-8-items.adoc
+++ b/doc/part-8-items.adoc
@@ -457,7 +457,7 @@ since it will probably be used multiple times. The constant
 ----
 /// heal by the given amount, without going over the maximum
 pub fn heal(&mut self, amount: i32) {
-    if let Some(mut fighter) = self.fighter {
+    if let Some(ref mut fighter) = self.fighter {
         fighter.hp += amount;
         if fighter.hp > fighter.max_hp {
             fighter.hp = fighter.max_hp;
diff --git a/src/bin/part-10-menu-saving.rs b/src/bin/part-10-menu-saving.rs
index 3b29b48..5cba8bc 100644
--- a/src/bin/part-10-menu-saving.rs
+++ b/src/bin/part-10-menu-saving.rs
@@ -206,7 +206,7 @@ impl Object {
 
     /// heal by the given amount, without going over the maximum
     pub fn heal(&mut self, amount: i32) {
-        if let Some(mut fighter) = self.fighter {
+        if let Some(ref mut fighter) = self.fighter {
             fighter.hp += amount;
             if fighter.hp > fighter.max_hp {
                 fighter.hp = fighter.max_hp;
diff --git a/src/bin/part-11-dungeon-progression.rs b/src/bin/part-11-dungeon-progression.rs
index 9cdac7c..cd60b48 100644
--- a/src/bin/part-11-dungeon-progression.rs
+++ b/src/bin/part-11-dungeon-progression.rs
@@ -221,7 +221,7 @@ impl Object {
 
     /// heal by the given amount, without going over the maximum
     pub fn heal(&mut self, amount: i32) {
-        if let Some(mut fighter) = self.fighter {
+        if let Some(ref mut fighter) = self.fighter {
             fighter.hp += amount;
             if fighter.hp > fighter.max_hp {
                 fighter.hp = fighter.max_hp;
diff --git a/src/bin/part-12-monster-item-progression.rs b/src/bin/part-12-monster-item-progression.rs
index c61df56..f9b420e 100644
--- a/src/bin/part-12-monster-item-progression.rs
+++ b/src/bin/part-12-monster-item-progression.rs
@@ -219,7 +219,7 @@ impl Object {
 
     /// heal by the given amount, without going over the maximum
     pub fn heal(&mut self, amount: i32) {
-        if let Some(mut fighter) = self.fighter {
+        if let Some(ref mut fighter) = self.fighter {
             fighter.hp += amount;
             if fighter.hp > fighter.max_hp {
                 fighter.hp = fighter.max_hp;
diff --git a/src/bin/part-13-adventure-gear.rs b/src/bin/part-13-adventure-gear.rs
index 827cf2a..2a1b65c 100644
--- a/src/bin/part-13-adventure-gear.rs
+++ b/src/bin/part-13-adventure-gear.rs
@@ -222,7 +222,7 @@ impl Object {
     /// heal by the given amount, without going over the maximum
     pub fn heal(&mut self, amount: i32, game: &Game) {
         let max_hp = self.max_hp(game);
-        if let Some(mut fighter) = self.fighter {
+        if let Some(ref mut fighter) = self.fighter {
             fighter.hp += amount;
             if fighter.hp > max_hp {
                 fighter.hp = max_hp;
diff --git a/src/bin/part-8-items.rs b/src/bin/part-8-items.rs
index 3617190..64469b6 100644
--- a/src/bin/part-8-items.rs
+++ b/src/bin/part-8-items.rs
@@ -191,7 +191,7 @@ impl Object {
 
     /// heal by the given amount, without going over the maximum
     pub fn heal(&mut self, amount: i32) {
-        if let Some(mut fighter) = self.fighter {
+        if let Some(ref mut fighter) = self.fighter {
             fighter.hp += amount;
             if fighter.hp > fighter.max_hp {
                 fighter.hp = fighter.max_hp;
diff --git a/src/bin/part-9-spells.rs b/src/bin/part-9-spells.rs
index 4b329e2..dc6c8e0 100644
--- a/src/bin/part-9-spells.rs
+++ b/src/bin/part-9-spells.rs
@@ -203,7 +203,7 @@ impl Object {
 
     /// heal by the given amount, without going over the maximum
     pub fn heal(&mut self, amount: i32) {
-        if let Some(mut fighter) = self.fighter {
+        if let Some(ref mut fighter) = self.fighter {
             fighter.hp += amount;
             if fighter.hp > fighter.max_hp {
                 fighter.hp = fighter.max_hp;