"Hangover Part II” qualifies as a valuable film as well, albeit in what it teaches us about the perils of formula, sequelitis and taking your audience for granted.Summer wouldn’t be fittingly launched without a go-for-broke raunchy comedy, the kind of uncensored, emotionally expansive movie where pleasure can be found not just in the taboos it gleefully smashes but in its celebration of friendship, emotional growth and sundry humanist values.A veritable textbook case of what not to do when re-making a comedy that depended on sheer surprise for most of its appeal, “The Hangover Part II” also inadvertently points up the weaknesses of the first movie, which became a huge hit two summers ago. Fans of “The Hangover” which starred Bradley Cooper, Ed Helms and Zach Galifianakis as refugees from a Las Vegas bachelor party gone horribly wrong — were prone to forget the long, laugh-free sequences in between the more memorable set pieces, just as they easily overlook the fact that, were it not for Galifianakis’s quietly deranged presence, “The Hangover” would likely have been a hamhanded, unevenly paced dud.Once the posse lands in Thailand, “just one beer” two nights before the wedding turns into a similar bender, with the three principals awaking in a Bangkok hotel room with a severed finger, a monkey in a Rolling Stones vest and no memory of how they got there. And they’ve managed to lose Stu’s future brother-in-law, launching a search of the city’s bars, strip clubs and tattoo parlors where they’ve left a trail of havoc in their wake.The one cheering exception to an otherwise ho-hum exercise is Galifianakis, who plays the spoiled, sartorially challenged man-child Alan, a preening, glowering, utterly fascinating conundrum composed of co-equal parts naivete, hauteur, feral menace and barely submerged homoerotic desire. It’s Alan — who describes himself as a “stay-at-home-son” with the gravitas of someone announcing he’s the head of the SEC — who has the film’s most surreally humorous dialogue. And it’s Galifianakis who gives those jokes extra mileage by way of the surreal afterthought. “I wish monkeys could Skype” is a funny line, sure. But it’s the wistful yearning with which Galifianakis adds, “Maybe one day” that sends it into the ether of comic genius.