Method headMap

guava/src/com/google/common/collect/Maps.java:986–989

(@ParametricNullness K toKey)

    }

    @Override
    public SortedMap<K, V> headMap(@ParametricNullness K toKey) {
      return asMap(backingSet().headSet(toKey), function);
    }

    @Override
    public SortedMap<K, V> tailMap(@ParametricNullness K fromKey) {