diff --git a/src/main/java/pythagoras/d/MathUtil.java b/src/main/java/pythagoras/d/MathUtil.java
index a5b9308..e071828 100644
--- a/src/main/java/pythagoras/d/MathUtil.java
+++ b/src/main/java/pythagoras/d/MathUtil.java
@@ -34,7 +34,7 @@ public class MathUtil
      */
     public static int ifloor (double v) {
         int iv = (int)v;
-        return (v < 0f) ? ((iv == v || iv == Integer.MIN_VALUE) ? iv : (iv - 1)) : iv;
+        return (v >= 0f || iv == v || iv == Integer.MIN_VALUE) ? iv : (iv - 1);
     }
 
     /**
@@ -43,7 +43,7 @@ public class MathUtil
      */
     public static int iceil (double v) {
         int iv = (int)v;
-        return (v > 0f) ? ((iv == v || iv == Integer.MAX_VALUE) ? iv : (iv + 1)) : iv;
+        return (v <= 0f || iv == v || iv == Integer.MAX_VALUE) ? iv : (iv + 1);
     }
 
     /**
diff --git a/src/main/java/pythagoras/d/Matrix3.java b/src/main/java/pythagoras/d/Matrix3.java
index 7007626..153be0d 100644
--- a/src/main/java/pythagoras/d/Matrix3.java
+++ b/src/main/java/pythagoras/d/Matrix3.java
@@ -471,11 +471,8 @@ public class Matrix3 implements IMatrix3, Serializable
     }
 
     /**
-     * Inverts this matrix and places the result in the given object. This code is based on the
-     * examples in the <a href="http://www.j3d.org/matrix_faq/matrfaq_latest.html">Matrix and
-     * Quaternion FAQ</a>.
-     *
-     * @return a reference to the result matrix, for chaining.
+     * @{@inheritDoc} This code is based on the examples in the
+     * <a href="http://www.j3d.org/matrix_faq/matrfaq_latest.html">Matrix and Quaternion FAQ</a>.
      */
     @Override // from IMatrix3
     public Matrix3 invert (Matrix3 result) throws SingularMatrixException {
@@ -641,6 +638,11 @@ public class Matrix3 implements IMatrix3, Serializable
         return result.set(m00*vx + m10*vy, m01*vx + m11*vy);
     }
 
+    /**
+     * {@inheritDoc} This uses the iterative polar decomposition algorithm described by
+     * <a href="http://www.cs.wisc.edu/graphics/Courses/838-s2002/Papers/polar-decomp.pdf">Ken
+     * Shoemake</a>.
+     */
     @Override // from IMatrix3
     public double extractRotation () {
         // start with the contents of the upper 2x2 portion of the matrix
diff --git a/src/main/java/pythagoras/d/Matrix4.java b/src/main/java/pythagoras/d/Matrix4.java
index abf8f1a..8d53d0d 100644
--- a/src/main/java/pythagoras/d/Matrix4.java
+++ b/src/main/java/pythagoras/d/Matrix4.java
@@ -705,6 +705,10 @@ public final class Matrix4 implements IMatrix4, Serializable
         return invert(new Matrix4());
     }
 
+    /**
+     * {@inheritDoc} This code is based on the examples in the
+     * <a href="http://www.j3d.org/matrix_faq/matrfaq_latest.html">Matrix and Quaternion FAQ</a>.
+     */
     @Override // from IMatrix4
     public Matrix4 invert (Matrix4 result) throws SingularMatrixException {
         double m00 = this.m00, m10 = this.m10, m20 = this.m20, m30 = this.m30;
@@ -917,6 +921,11 @@ public final class Matrix4 implements IMatrix4, Serializable
         return extractRotation(new Quaternion());
     }
 
+    /**
+     * {@inheritDoc} This uses the iterative polar decomposition algorithm described by
+     * <a href="http://www.cs.wisc.edu/graphics/Courses/838-s2002/Papers/polar-decomp.pdf">Ken
+     * Shoemake</a>.
+     */
     @Override // from IMatrix4
     public Quaternion extractRotation (Quaternion result) throws SingularMatrixException {
         // start with the contents of the upper 3x3 portion of the matrix
diff --git a/src/main/java/pythagoras/d/Transforms.java b/src/main/java/pythagoras/d/Transforms.java
index 82649a9..a010797 100644
--- a/src/main/java/pythagoras/d/Transforms.java
+++ b/src/main/java/pythagoras/d/Transforms.java
@@ -31,8 +31,7 @@ public class Transforms
      * into} may refer to the same instance as {@code a} or {@code b}.
      * @return {@code into} for chaining.
      */
-    public static AffineTransform multiply (
-        AffineTransform a, AffineTransform b, AffineTransform into) {
+    public static <T extends Transform> T multiply (AffineTransform a, AffineTransform b, T into) {
         return multiply(a.m00, a.m01, a.m10, a.m11, a.tx, a.ty,
                         b.m00, b.m01, b.m10, b.m11, b.tx, b.ty, into);
     }
@@ -42,11 +41,9 @@ public class Transforms
      * into} may refer to the same instance as {@code a}.
      * @return {@code into} for chaining.
      */
-    public static AffineTransform multiply (
-        AffineTransform a, double m00, double m01, double m10, double m11, double tx, double ty,
-        AffineTransform into) {
-        return multiply(a.m00, a.m01, a.m10, a.m11, a.tx, a.ty,
-                        m00, m01, m10, m11, tx, ty, into);
+    public static <T extends Transform> T multiply (
+        AffineTransform a, double m00, double m01, double m10, double m11, double tx, double ty, T into) {
+        return multiply(a.m00, a.m01, a.m10, a.m11, a.tx, a.ty, m00, m01, m10, m11, tx, ty, into);
     }
 
     /**
@@ -54,27 +51,24 @@ public class Transforms
      * into} may refer to the same instance as {@code b}.
      * @return {@code into} for chaining.
      */
-    public static AffineTransform multiply (
-        double m00, double m01, double m10, double m11, double tx, double ty,
-        AffineTransform b, AffineTransform into) {
-        return multiply(m00, m01, m10, m11, tx, ty,
-                        b.m00, b.m01, b.m10, b.m11, b.tx, b.ty, into);
+    public static <T extends Transform> T multiply (
+        double m00, double m01, double m10, double m11, double tx, double ty, AffineTransform b, T into) {
+        return multiply(m00, m01, m10, m11, tx, ty, b.m00, b.m01, b.m10, b.m11, b.tx, b.ty, into);
     }
 
     /**
      * Multiplies the supplied two affine transforms, storing the result in {@code into}.
      * @return {@code into} for chaining.
      */
-    public static AffineTransform multiply (
+    public static <T extends Transform> T multiply (
         double am00, double am01, double am10, double am11, double atx, double aty,
-        double bm00, double bm01, double bm10, double bm11, double btx, double bty,
-        AffineTransform into) {
-        into.m00 = am00 * bm00 + am10 * bm01;
-        into.m01 = am01 * bm00 + am11 * bm01;
-        into.m10 = am00 * bm10 + am10 * bm11;
-        into.m11 = am01 * bm10 + am11 * bm11;
-        into.tx  = am00 *  btx + am10 *  bty + atx;
-        into.ty  = am01 *  btx + am11 *  bty + aty;
+        double bm00, double bm01, double bm10, double bm11, double btx, double bty, T into) {
+        into.setTransform(am00 * bm00 + am10 * bm01,
+                          am01 * bm00 + am11 * bm01,
+                          am00 * bm10 + am10 * bm11,
+                          am01 * bm10 + am11 * bm11,
+                          am00 *  btx + am10 *  bty + atx,
+                          am01 *  btx + am11 *  bty + aty);
         return into;
     }
 }
diff --git a/src/main/java/pythagoras/d/Vector.java b/src/main/java/pythagoras/d/Vector.java
index faf38a2..60b918f 100644
--- a/src/main/java/pythagoras/d/Vector.java
+++ b/src/main/java/pythagoras/d/Vector.java
@@ -79,6 +79,12 @@ public class Vector extends AbstractVector
         return add(x, y, this);
     }
 
+    /** Subtracts a vector in-place from this one.
+     * @return a reference to this vector, for chaining. */
+    public Vector subtractLocal (double x, double y) {
+        return subtract(x, y, this);
+    }
+
     /** Adds a scaled vector in-place to this one.
      * @return a reference to this vector, for chaining. */
     public Vector addScaledLocal (IVector other, double v) {