That's widely thought to be the most difficult form.

He is also known for geometrically complicated and technically difficult forms.

"This is an important new treatment option for patients who are fighting this very difficult form of cancer."

This is a very difficult form to fill in in relation to who's attending the meeting.

Historically an "S" is a very difficult form to forge.

"I have always thought that must be the most difficult form of art."

The Defense Department faces a particularly difficult form of this predicament.

Many children start out writing like that, with the most difficult form.

"These operations are a difficult form of surgery, and can last as long as 12 hours," he said.

Square baskets are the most difficult form (speaking only of shape) to make.