CHAIN

CHAIN is a notion that derived from the notion of chain and meant to generalize chains and expletive-argument pairs. The notion CHAIN thus covers two cases: chains and expletive-argument pairs.

Example
In (i) the expletive-argument pair (therei, a mani) which is not a chain, constitutes a CHAIN, because there binds a man and the pair behaves as a chain with regard to the visibility condition.

(i) Therei is a mani in the room

Link
Utrecht Lexicon of Linguistics