so let's get a firm definition of solid vs "internally solid." IMV that would be any holding that has no trump losers more than 50% of the time across the whole spectrum of partner's possible trump holding (assuming no ruffs). For example using this definition a trump holding of AKJ5432 ought to be good enough as with 4+ cards there are no losers, with 3 cards at least 75%, with 2 cards somewhere in the greater than 50% range, with 1 card need 32 break and finesse , and finally with a void you are in "deep doo doo".