code: public final <V> ImmutableMap<E, V> toMap(Function<? super E, V> valueFunction) {
      return Maps.toMap(getDelegate(), valueFunction);
    }
docs: Returns an immutable map whose keys are the distinct elements of this {@code FluentIterable}
  and whose value for each key was computed by {@code valueFunction}. The map's iteration order
  is the order of the first appearance of each key in this iterable.
  
  <p>When there are multiple instances of a key in this iterable, 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><b>{@code Stream} equivalent:</b> {@code stream.collect(ImmutableMap.toImmutableMap(k -> k,
  valueFunction))}.
  
  @throws NullPointerException if any element of this iterable is {@code null}, or if {@code
  valueFunction} produces {@code null} for any key
  @since 14.0