np/NP -> det/NP, nbar/NP.
is postulated. In general, a chain rule is used bottom-up from its semantic head and top-down on the non-semantic-head siblings. Thus, if a non-semantic-head subconstituent has the same semantics as the left-hand-side, a recursive top-down generation with the same semantics will be invoked. In theory, this can lead to nontermination, unless syntactic factors eliminate the recursion, as they would in the rule above regardless of which element is chosen as semantic head. In a rule for relative clause introduction such as the following (in highly abbreviated form)
nbar/N -> nbar/N, sbar/N.
we can (and must) choose the nominal as semantic head to effect termination. However, there are other problematic cases, such as verb-movement analyses of verb-second languages, whose detailed discussion is beyond the scope of this paper.