code: public <R> R into(Function<? super _1, ? extends R> fn) {
          return fn.apply(head());
      }
docs: Apply {@link SingletonHList#head()} to <code>fn</code> and return the result.
  
  @param fn  the function to apply
  @param <R> the return type of the function
  @return the result of applying the head to the function