android/guava/src/com/google/common/collect/ImmutableSetMultimap.java
public static <K, V> ImmutableSetMultimap<K, V> copyOf(
      Multimap<? extends K, ? extends V> multimap) {
    return copyOf(multimap, null);
  }
public static <K, V> ImmutableSetMultimap<K, V> copyOf(
      Multimap<? extends K, ? extends V> multimap) {
    return copyOf(multimap, null);
  }
Returns an immutable set multimap containing the same mappings as {@code multimap}. The
generated multimap's key and value orderings correspond to the iteration ordering of the {@code
multimap.asMap()} view. Repeated occurrences of an entry in the multimap after the first are
ignored.

<p>Despite the method name, this method attempts to avoid actually copying the data when it is
safe to do so. The exact circumstances under which a copy will or will not be performed are
undocumented and subject to change.

@throws NullPointerException if any key or value in {@code multimap} is null