Found:

Empty Category Principle

SYNTAX: a principle which requires that empty categories be properly governed. It is assumed that the ECP does not hold of all empty categories: it holds for A- and A'-bound traces (i.e. NP-traces and variables), but not for pronominal empty categories like pro and PRO. Well-known examples of ECP-violations are extractions of an adjunct out of an island, as in (i) (containing a wh-island) and configurations displaying the that-trace effect in English, as in (ii). In both cases, the trace cannot be properly antecedent-governed because of the intervention of a barrier.

(i)  * howi did John ask [ whether Bill fixed the car ti ]
(ii) * whoi does John believe [ that ti will fix the car ]