The constituency-based trees are the a-trees on the left, and the dependency-based trees the b-trees on the right. Certainly aspects of these trees - especially of the constituency trees - can be disputed. In the current context, the trees are intended merely to suggest by way of illustration how control and PRO are conceived of. The indices are a common means of identifying PRO and with its antecedent in the control predicate, and the orange arrows indicate further the control relation. In a sense, the controller assigns its index to PRO, which identifies the argument that is understood as the subject of the subordinate predicate.