--- /dev/null	Thu Jan 01 00:00:00 1970 +0000
+++ b/src/main/java/fj/data/fingertrees/Single.java	Fri Mar 20 21:04:03 2015 +0900
@@ -0,0 +1,83 @@
+package fj.data.fingertrees;
+
+import fj.F;
+import fj.P2;
+import static fj.P.p;
+
+/**
+ * A tree with a single element.
+ */
+public final class Single<V, A> extends FingerTree<V, A> {
+  private final A a;
+  private final V v;
+
+  Single(final Measured<V, A> m, final A a) {
+    super(m);
+    this.a = a;
+    v = m.measure(a);
+  }
+
+  @Override public <B> B foldRight(final F<A, F<B, B>> aff, final B z) {
+    return aff.f(a).f(z);
+  }
+
+  @Override public A reduceRight(final F<A, F<A, A>> aff) {
+    return a;
+  }
+
+  @Override public <B> B foldLeft(final F<B, F<A, B>> bff, final B z) {
+    return bff.f(z).f(a);
+  }
+
+  @Override public A reduceLeft(final F<A, F<A, A>> aff) {
+    return a;
+  }
+
+  @Override public <B> FingerTree<V, B> map(final F<A, B> abf, final Measured<V, B> m) {
+    return new Single<V, B>(m, abf.f(a));
+  }
+
+  /**
+   * Returns the annotation of this tree's single element.
+   *
+   * @return the annotation of this tree's single element.
+   */
+  public V measure() {
+    return v;
+  }
+
+  /**
+   * Pattern matching on the structure of this tree. Matches the singleton tree.
+   */
+  @Override public <B> B match(final F<Empty<V, A>, B> empty, final F<Single<V, A>, B> single,
+                               final F<Deep<V, A>, B> deep) {
+    return single.f(this);
+  }
+
+  @Override public FingerTree<V, A> cons(final A b) {
+    final MakeTree<V, A> mk = mkTree(measured());
+    return mk.deep(mk.one(b), new Empty<V, Node<V, A>>(measured().nodeMeasured()), mk.one(a));
+  }
+
+  @Override public FingerTree<V, A> snoc(final A b) {
+    final MakeTree<V, A> mk = mkTree(measured());
+    return mk.deep(mk.one(a), new Empty<V, Node<V, A>>(measured().nodeMeasured()), mk.one(b));
+  }
+
+  @Override public FingerTree<V, A> append(final FingerTree<V, A> t) {
+    return t.cons(a);
+  }
+
+  @Override public P2<Integer, A> lookup(final F<V, Integer> o, final int i) {
+    return p(i, a);
+  }
+
+  /**
+   * Returns the single element of this tree.
+   *
+   * @return the single element of this tree.
+   */
+  public A value() {
+    return a;
+  }
+}