diff --git a/classes/private/Confidential.java b/classes/private/Confidential.java
index 99eeefd..6dc740d 100644
--- a/classes/private/Confidential.java
+++ b/classes/private/Confidential.java
@@ -1,9 +1,8 @@
 public class Confidential {
 
-	private String secret = "";
-	private String hidden = "burn after reading";
+    private String secret = "";
 
-	public Confidential(String text) {
-		this.secret = text.toUpperCase();
-	}
+    public Confidential(String text) {
+        secret = text.toUpperCase();
+    }
 }
diff --git a/classes/private/Expose.java b/classes/private/Expose.java
index 8a52bb3..dac57fe 100644
--- a/classes/private/Expose.java
+++ b/classes/private/Expose.java
@@ -2,24 +2,24 @@ import java.lang.reflect.Field;
 
 public class Expose {
 
-	public static void main(String[] args) {
-		Confidential message = new Confidential("top secret text");
-		Field secretField = null;
-		try {
-			secretField = Confidential.class.getDeclaredField("secret");
-		}
-		catch (NoSuchFieldException e) {
-			System.err.println(e);
-			System.exit(1);
-		}
-		secretField.setAccessible(true); // break the lock!
-		try {
-			String wasHidden = (String) secretField.get(message);
-			System.out.println("message.secret = " + wasHidden);
-		}
-		catch (IllegalAccessException e) { 
-			// this will not happen after setAcessible(true)
-			System.err.println(e);
-		}	
-	}
+    public static void main(String[] args) {
+        Confidential message = new Confidential("top secret text");
+        Field secretField = null;
+        try {
+            secretField = Confidential.class.getDeclaredField("secret");
+        }
+        catch (NoSuchFieldException e) {
+            System.err.println(e);
+            System.exit(1);
+        }
+        secretField.setAccessible(true); // break the lock!
+        try {
+            String wasHidden = (String) secretField.get(message);
+            System.out.println("message.secret = " + wasHidden);
+        }
+        catch (IllegalAccessException e) { 
+            // this will not happen after setAcessible(true)
+            System.err.println(e);
+        }   
+    }
 }
diff --git a/classes/vector2d_v3_prophash.py b/classes/vector2d_v3_prophash.py
new file mode 100644
index 0000000..2eca4ec
--- /dev/null
+++ b/classes/vector2d_v3_prophash.py
@@ -0,0 +1,161 @@
+"""
+A 2-dimensional vector class
+
+    >>> v1 = Vector2d(3, 4)
+    >>> print(v1.x, v1.y)
+    3.0 4.0
+    >>> x, y = v1
+    >>> x, y
+    (3.0, 4.0)
+    >>> v1
+    Vector2d(3.0, 4.0)
+    >>> v1_clone = eval(repr(v1))
+    >>> v1 == v1_clone
+    True
+    >>> print(v1)
+    (3.0, 4.0)
+    >>> octets = bytes(v1)
+    >>> octets
+    b'd\\x00\\x00\\x00\\x00\\x00\\x00\\x08@\\x00\\x00\\x00\\x00\\x00\\x00\\x10@'
+    >>> abs(v1)
+    5.0
+    >>> bool(v1), bool(Vector2d(0, 0))
+    (True, False)
+
+
+Test of ``.frombytes()`` class method:
+
+    >>> v1_clone = Vector2d.frombytes(bytes(v1))
+    >>> v1_clone
+    Vector2d(3.0, 4.0)
+    >>> v1 == v1_clone
+    True
+
+
+Tests of ``format()`` with Cartesian coordinates:
+
+    >>> format(v1)
+    '(3.0, 4.0)'
+    >>> format(v1, '.2f')
+    '(3.00, 4.00)'
+    >>> format(v1, '.3e')
+    '(3.000e+00, 4.000e+00)'
+
+
+Tests of the ``angle`` method::
+
+    >>> Vector2d(0, 0).angle()
+    0.0
+    >>> Vector2d(1, 0).angle()
+    0.0
+    >>> epsilon = 10**-8
+    >>> abs(Vector2d(0, 1).angle() - math.pi/2) < epsilon
+    True
+    >>> abs(Vector2d(1, 1).angle() - math.pi/4) < epsilon
+    True
+
+
+Tests of ``format()`` with polar coordinates:
+
+    >>> format(Vector2d(1, 1), 'p')  # doctest:+ELLIPSIS
+    '<1.414213..., 0.785398...>'
+    >>> format(Vector2d(1, 1), '.3ep')
+    '<1.414e+00, 7.854e-01>'
+    >>> format(Vector2d(1, 1), '0.5fp')
+    '<1.41421, 0.78540>'
+
+# BEGIN VECTOR2D_V3_DEMO
+Tests of `x` and `y` read-only properties:
+
+    >>> v1.x, v1.y
+    (3.0, 4.0)
+    >>> v1.x = 123
+    Traceback (most recent call last):
+      ...
+    AttributeError: can't set attribute
+
+# END VECTOR2D_V3_HASH_DEMO
+
+Tests of hashing:
+# BEGIN VECTOR2D_V3_HASH_DEMO
+
+    >>> v1 = Vector2d(3, 4)
+    >>> v2 = Vector2d(3.1, 4.2)
+    >>> hash(v1), hash(v2)
+    (7, 384307168202284039)
+    >>> len(set([v1, v2]))
+    2
+
+# END VECTOR2D_V3_DEMO
+
+"""
+
+from array import array
+import math
+
+# BEGIN VECTOR2D_V3_PROP
+class Vector2d:
+    typecode = 'd'
+
+    def __init__(self, x, y):
+        self.__x = float(x)  # <1>
+        self.__y = float(y)
+
+    @property  # <2>
+    def x(self):  # <3>
+        return self.__x  # <4>
+
+    @property  # <5>
+    def y(self):
+        return self.__y
+
+    def __iter__(self):
+        return (i for i in (self.x, self.y))  # <6>
+
+    # remaining methods follow (omitted in book listing)
+# END VECTOR2D_V3_PROP
+
+    def __repr__(self):
+        class_name = type(self).__name__
+        return '{}({!r}, {!r})'.format(class_name, *self)
+
+    def __str__(self):
+        return str(tuple(self))
+
+    def __bytes__(self):
+        return (bytes([ord(self.typecode)]) +
+                bytes(array(self.typecode, self)))
+
+    def __eq__(self, other):
+        return tuple(self) == tuple(other)
+
+# BEGIN VECTOR_V3_HASH
+    def __hash__(self):
+        return hash(self.x) ^ hash(self.y)
+# END VECTOR_V3_HASH
+
+    def __abs__(self):
+        return math.hypot(self.x, self.y)
+
+    def __bool__(self):
+        return bool(abs(self))
+
+    def angle(self):
+        return math.atan2(self.y, self.x)
+
+    def __format__(self, fmt_spec=''):
+        if fmt_spec.endswith('p'):
+            fmt_spec = fmt_spec[:-1]
+            coords = (abs(self), self.angle())
+            outer_fmt = '<{}, {}>'
+        else:
+            coords = self
+            outer_fmt = '({}, {})'
+        components = (format(c, fmt_spec) for c in coords)
+        return outer_fmt.format(*components)
+
+    @classmethod
+    def frombytes(cls, octets):
+        typecode = chr(octets[0])
+        memv = memoryview(octets[1:]).cast(typecode)
+        return cls(*memv)