That can be difficult, although the political landscape is changing on these issues.
To generalize from a single example is difficult, especially when the example itself is changing, growing, and evolving.
But the firm said market conditions were difficult for fashion retailers as digitisation is changing both shoppers' behaviour and the way fashion companies operate.
Upgrading is difficult, expensive and time consuming, but this is changing.
"In tech, market definitions are difficult because companies are changing so fast, and that makes antitrust a blunt tool," said Ed Black, president of the Computer & Communications Industry Association, a trade group that supported the Justice Department's case against Microsoft.
However, given the consensus among scientists that climate is changing, it is a difficult assumption to defend at longer time scales, such as a century, or the 50 80 year rotation length of managed, even-aged stands in the U.S. Northwest.
China is changing so fast, it's probably difficult to make that call.
It was difficult because the ball was changing hands that many times, I'd be surprised if there were three or four [consecutive] passes in the whole match.
Talking is easy, changing is more difficult but nevertheless we have to do it".
"The reason it's difficult," he said, "is because I'm changing all the time".
