operator

The stories did not seem to me to touch life. […] They left me with the impression of a well-delivered stereopticon lecture, with characters about as life-like as the shadows on the screen, and whisking on and off, at the mercy of the operator.

Francis Urquhart: I think Lord Billsborough is starting to lose touch a bit.

Tim Stamper: Shame. Used to be a hell of an operator in his day.

In the sentence "What did Bill say he wants to buy?", "what" is an operator, binding a phonetically empty variable.