Overt Comp constraint on stacked relatives

Stacked relatives of the kind in ((229)) are handled.

(228)0(228

(229)

[[the book [that Bill likes]] [which Mary wrote]] 

There is a constraint on stacked relatives: all but the relative clause closest to the head-NP must have either an overt Comp or an overt NP_w. Thus ((230)) is ungrammatical.

(229)0(229

(230)

*[[the book [that Bill likes]] [Mary wrote]] 

Again, no good way of handling this constraint is known to us currently.