In a non-configurational language such as German, which features argument composition, heads combine with complements that are not their own arguments but rather those of a predicate they compose with, i.e., they need to cater for unknown raised arguments by means of underspecified valence lists.
... weil ein Buch 2 er 1 ihm 2 zu kaufen 2 versprach 1 'because he 1 promised 1 to buy him 2 a book 2 .