* the first appearance of each key in {@code keys}.
   *
   * <p>When there are multiple instances of a key in {@code keys}, it is unspecified whether {@code
   * valueFunction} will be applied to more than one instance of that key and, if it is, which
   * result will be mapped to that key in the returned map.
   *
   * <p>If {@code keys} is a {@link Set}, a live view can be obtained instead of a copy using {@link
   * Maps#asMap(Set, Function)}.