The Grammar-driven Kernel Definition: Given the two improvements discussed above, we can define the new kernel by beginning with the feature vector representation of a parse tree T as follows: where # subtree i (T) is the occurrence number of the i th sub-tree type (subtree i ) in T. Please note that, different from the previous tree kernel, here we loosen the condition for the occurrence of a subtree by allowing both original and reduced rules (Improvement 1) and node feature mutations (Improvement 2).
we modify the criteria by which a subtree is said to occur.