6. The method of claim 5 , further comprising: determining whether the OP node satisfies subtree constraints of a tree pattern of the result; if not, then determining that no unique matching is possible and that no candidate match can be derived; and if so, then determining that a tree pattern of the result has a constraint on an order.
5. The method of claim 4 , further comprising: if the same candidate match is not returned by multiple OP node children, then determining that no unique matching is possible and that no candidate match can be derived; and if the same candidate match is returned by multiple OP node children, then assembling a result by concatenating candidate matches for each of the OP node children.