|1.||Who did Mary ask __ to meet John?|
|2.||Who did Mary ask John to meet __?|
|3.||Which book did you persuade John to read __ during the holiday?|
|4.||What did you pack __ to take __ on holiday?|
|5.||Which book did the choir sing that carol from __?|
Note that in all cases apart from 4 there is one filler (the wh-word or -phrase, as underlined) and one gap location (marked by the underscore).
In the case of 4, the same filler is linked to two positions, one showing what 'you' packed, and the other showing what 'you' took on holiday.