17. The system according to claim 14 , wherein said operation specification comprises a completion condition described by a specific state of said metadata; and wherein said processor device is further configured to: apply a completion mark, among said abstract documents that satisfy said completion condition, to reverse abstract documents that arrived at said graph structure in reverse order; and determine verification failure if said reverse abstract document is without an applied completion mark.
18. The system according to claim 17 , wherein said processor device is further configured to: present, among initial abstract documents, a path going up to said abstract document without said applied completion mark as a failure example.