android/guava/src/com/google/common/collect/ImmutableSortedMap.java
@SuppressWarnings("unchecked")
  public static <K extends Comparable<? super K>, V> ImmutableSortedMap<K, V> of(
      K k1, V v1, K k2, V v2) {
    return ofEntries(entryOf(k1, v1), entryOf(k2, v2));
  }
@SuppressWarnings("unchecked")
  public static <K extends Comparable<? super K>, V> ImmutableSortedMap<K, V> of(
      K k1, V v1, K k2, V v2) {
    return ofEntries(entryOf(k1, v1), entryOf(k2, v2));
  }
Returns an immutable sorted map containing the given entries, sorted by the natural ordering of
their keys.

@throws IllegalArgumentException if the two keys are equal according to their natural ordering