But that of women is a particularly tricky category.
"I won't stop saying it: this is a particularly tricky group.
It's a particularly tricky time, that moment between dependence and independence.
And if there's a particularly tricky scene I'm worried about, I'll do it before then as well.
Worse, says Steven Law, the president of both American Crossroads and Crossroads GPS, Mr Obama is a particularly tricky man to tear down.
"This is a particularly tricky time -- we've never seen anything quite like it," said Christopher Burge, Christie's honorary chairman in America, a 31-year veteran of auction houses and the company's principal auctioneer.
It's a particularly tricky topic for historians of science and medicine, because those subjects are generally thought to progress over time, not just change (as 'art' or 'politics' might).
But finding images that could represent all of these books, and make them work as a series, is a particularly tricky problem, because there are three distinct stages to Calvino's writing: a realist phase, a fantastical phase, and a kind of semiotic or metafictional phase.
Justine Roberts, founder of Mumsnet, said: "How children negotiate the internet is a particularly tricky parenting challenge as it's not one that most of us have experienced in our own childhoods, so sometimes it's hard to know just how concerned to be.
The Spaniard said at the time that he was surprised some of his rivals were not testing because it was a particularly tricky track to learn.
