We discussed the advantages of treating a subordinator as a head vs. an adverb. We agreed that when it occurs in the subord clause, the user can decide if it's a head (attaching to S) or a modifier (attaching to just VP or VP and S). However, when it's in the matrix clause, it should always be an adverb (an the user can indicate if it attaches just to S, VP or to VP and S). This has two advantages: first, we don't have to make the initial symbol compatible with CP, and second we don't have to distinguish between these Matrix CPs and those created by the yes/no analysis for "ma" in Mandarin.
Further constraint of the adverb order must be added by the user in TDL