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The infinitive to is considered an auxiliary verb in the XTAG system, and
selects the auxiliary tree in Figure 21.1. To, like do, does
not interact with the other auxiliary verbs, adjoining only to main verb base
forms, and carrying infinitive mode. It is used in embedded clauses, both with
and without a complementizer, as in sentences ((406))-((408)). Since it
cannot be inverted, it simply does not select the trees in
- John wants to have a backpack .
- John wants Mary to have a backpack .
- John wants for Mary to have a backpack .
The usage of infinitival to interacts closely with the distribution of
null subjects (PRO), and is described in more detail in