Given a tree t, the set S(t) is defined as the set containing all the relevant CSSTs of the tree t. As for the tree kernels (Collins and Duffy, 2002), the set S(t) contains all CSSTs derived from the subtrees of t such that if a node n belongs to a subtree t s , all the siblings of n in t belongs to t s .
productions of the initial subtrees are complete.