X: X-ist?

Sargent (not Tom) was immortalized for his portrait of Madame X. Roentgen discovered the X-ray, and had an element named after him. No such luck for Leibenstein: his concept of X-efficiency (or is it X-inefficiency?) never had much of a following. Stigler even questeioned its X-istence. So, for now, the field is still wide open. Dear fellow Chinese economists, this is a slot in which we have a comparative advantage. Nominations are welcome.