'Return a slice object that corresponds to the
range of values that defines the subtree which has the
element with index *begin* as its root.'

def searchSubtree(self, begin):

    end = (begin + 1)
    total = self[begin].arity
    while (total > 0):
        total += (self[end].arity - 1)
        end += 1
    return slice(begin, end)
