Attempts to overcome this difficulty are based on a greedy approach which starts with firstorder dependencies between labels and iteratively increases the scope of dependency patterns under the constraint that a high-order dependency is selected only if it extends an existing lower order feature (Müller et al., 2013).
feature selection may only choose only few higherorder features, motivating the need for an effective variable-order CRF (voCRF) training procedure (Ye et al., 2009).