The Collins parser divides a constituent into a non-terminal head, non-terminals that are on the left of the head, and non-terminals that are on the right of the head. Abstractly speaking, LHS=L_n L_{n-1}...L_1 H R_1 ...R_{n-1}R_n, where $L_i$, $H$, and $R_i$ are symbols representing its category, head, and head tag, such as VP(dumped, VP) in the example sentence “Workers dumped sacks into a bin.”