In our observation, a complete span functions as a relatively independent and complete semantic structure in the parse tree, we thus believe that it can provide sufficient information to decide the head of a subtree without looking at any larger context.
for each node in a candidate parse, its sibling subtree features is the collection of all 3-tuples: where h represents the word form, the Part-of-Speech tag, or the combination of the word form and the Part-of-Speech tag of the head node of m; s is the nearest sibling node of m in-between h and m; and the expression ( ) represents the feature encoded on a trimmed subtree in the set ( ), such that the trimmed subtree is the one extracted within the local window .