Since its introduction in 1987, PCK has become a widely useful and used notion.

Oh, to be sure, once in a great while some useful notion is put forth.

Besides, we've already picked up several useful notions from the Graysons.

Likewise, he recognized zero as useful notion, corresponding to the null class, but not as a number.

This steelworks is only partially automated, and you might get some useful notions on how to improve production.

Kim strained her brain, but could not squeeze out any useful notion.

Matrices are a useful notion to encode linear maps.

However, while this is a useful notion in the case of total orders, it does not tell us much in the case of partially ordered sets.

We now introduce some useful notions.

Indeed this is the more useful notion in singularity theory.