diff --git a/src/main/java/pythagoras/d/Matrix3.java b/src/main/java/pythagoras/d/Matrix3.java
index 9fdc3d0..d7ba7fe 100644
--- a/src/main/java/pythagoras/d/Matrix3.java
+++ b/src/main/java/pythagoras/d/Matrix3.java
@@ -18,7 +18,7 @@ public class Matrix3 implements IMatrix3, Serializable
     /** the identity matrix. */
     public static final Matrix3 IDENTITY = new Matrix3();
 
-    /** The values of the matrix. The names take the form {@mCOLROW}. */
+    /** The values of the matrix. The names take the form {@code mCOLROW}. */
     public double m00, m10, m20;
     public double m01, m11, m21;
     public double m02, m12, m22;
diff --git a/src/main/java/pythagoras/d/Matrix4.java b/src/main/java/pythagoras/d/Matrix4.java
index 1d83b09..23f51c9 100644
--- a/src/main/java/pythagoras/d/Matrix4.java
+++ b/src/main/java/pythagoras/d/Matrix4.java
@@ -21,7 +21,7 @@ public final class Matrix4 implements IMatrix4, Serializable
     /** An empty matrix array. */
     public static final Matrix4[] EMPTY_ARRAY = new Matrix4[0];
 
-    /** The values of the matrix. The names take the form {@mCOLROW}. */
+    /** The values of the matrix. The names take the form {@code mCOLROW}. */
     public double m00, m10, m20, m30;
     public double m01, m11, m21, m31;
     public double m02, m12, m22, m32;
diff --git a/src/main/java/pythagoras/d/Transform.java b/src/main/java/pythagoras/d/Transform.java
index 7dfe8e1..73b448e 100644
--- a/src/main/java/pythagoras/d/Transform.java
+++ b/src/main/java/pythagoras/d/Transform.java
@@ -4,6 +4,8 @@
 
 package pythagoras.d;
 
+import pythagoras.util.NoninvertibleTransformException;
+
 /**
  * Represents a geometric transform. Specialized implementations exist for identity, rigid body,
  * uniform, non-uniform, and affine transforms.
diff --git a/src/main/java/pythagoras/f/Matrix3.java b/src/main/java/pythagoras/f/Matrix3.java
index 3eca82f..3da4487 100644
--- a/src/main/java/pythagoras/f/Matrix3.java
+++ b/src/main/java/pythagoras/f/Matrix3.java
@@ -18,7 +18,7 @@ public class Matrix3 implements IMatrix3, Serializable
     /** the identity matrix. */
     public static final Matrix3 IDENTITY = new Matrix3();
 
-    /** The values of the matrix. The names take the form {@mCOLROW}. */
+    /** The values of the matrix. The names take the form {@code mCOLROW}. */
     public float m00, m10, m20;
     public float m01, m11, m21;
     public float m02, m12, m22;
diff --git a/src/main/java/pythagoras/f/Matrix4.java b/src/main/java/pythagoras/f/Matrix4.java
index b06286d..42e6305 100644
--- a/src/main/java/pythagoras/f/Matrix4.java
+++ b/src/main/java/pythagoras/f/Matrix4.java
@@ -21,7 +21,7 @@ public final class Matrix4 implements IMatrix4, Serializable
     /** An empty matrix array. */
     public static final Matrix4[] EMPTY_ARRAY = new Matrix4[0];
 
-    /** The values of the matrix. The names take the form {@mCOLROW}. */
+    /** The values of the matrix. The names take the form {@code mCOLROW}. */
     public float m00, m10, m20, m30;
     public float m01, m11, m21, m31;
     public float m02, m12, m22, m32;
diff --git a/src/main/java/pythagoras/f/Transform.java b/src/main/java/pythagoras/f/Transform.java
index 3fe1fd3..1e33a6d 100644
--- a/src/main/java/pythagoras/f/Transform.java
+++ b/src/main/java/pythagoras/f/Transform.java
@@ -4,6 +4,8 @@
 
 package pythagoras.f;
 
+import pythagoras.util.NoninvertibleTransformException;
+
 /**
  * Represents a geometric transform. Specialized implementations exist for identity, rigid body,
  * uniform, non-uniform, and affine transforms.