code: @Nonnull
    public TypeaheadPrefetch setTTL (@Nonnegative final int nTTL)
    {
      if (nTTL < 1)
        throw new IllegalArgumentException ("TTL is too small: " + nTTL);
      m_nTTL = nTTL;
      return this;
    }
docs: The time (in milliseconds) the prefetched data should be cached in
  localStorage. Defaults to <code>86400000</code> (1 day).
  
  @param nTTL
  Time to live in milliseconds. Must be &ge; 1.
  @return this