In general, we can stack more interaction functions and BiLSTMs to enable deeper reasoning over the dependency trees.
the hid- den states of the (l + 1)-th layer H (l+1) can be calculated from the hidden state of the previous layer H (l) : where p i indicates the parent index of the word represents the interaction functions between the hidden state at the i-th and p ith positions under the dependency edges ( The number of layers L can be chosen according to the performance on the development set.