21.   For evening entertainment there is a piano bar and, for the more energetic, a sound-proofed discotheque operates in the basement.

22.   Genes operate together in determining the characteristics of an individual organism.

23.   He had the revolutionary idea that gravity was not just a force that operated in a fixed background of space-time.

24.   He started thinking about Careta, who he knew was in with this car-theft ring that operated in the area.

25.   Here the Market mechanism appears to operate in a relatively straight forward way, but there are a number of problems.

26.   His left shoulder, which will be operated on in December, was painful, and his confidence shot to pieces.

27.   I discovered that it is true that a serial killer operated in the area.

28.   If this were so, kin selection could operate only in species of high intelligence.

29.   In international trade, however, the bill of exchange still operates in this way.