code: public static Pair<String, String> internedStringPair(String first, String second) {
      return new MutableInternedPair(first, second);
    }
docs: Returns an MutableInternedPair where the Strings have been interned.
  This is a factory method for creating an
  MutableInternedPair.  It requires the arguments to be Strings.
  If this Pair is serialized
  and then deserialized, first and second are interned upon
  deserialization.
  <p><i>Note:</i> I put this in thinking that its use might be
  faster than calling <code>x = new Pair(a, b).stringIntern()</code>
  but it's not really clear whether this is true.
  
  @param first  The first object
  @param second The second object
  @return An MutableInternedPair, with given first and second