2. The system according to claim 1 , wherein said constraint rule is selected from the group consisting of: (i) searching said minimal tree containing said element being queried, wherein an anchor referred to by said element being queried can be limited to an anchor from said minimal tree; (ii) searching said minimal tree containing said element being queried, wherein said anchor referred to by said element being queried can be limited to said anchor from said same tree and said anchor referred to by said element being queried is a leaf node of an anchor tree; and (iii) searching said minimal tree containing said element being queried, wherein said anchor referred to by said element is from said anchor tree and said anchor referred to by said element is a leaf node of said anchor tree.
3. The system according to claim 2 , further comprising a query evaluating unit for constraining said query from said client computer on the basis of said constraint rules configured by said constraint configuring unit and said first and second anchors and said nested relationship between said first and second anchors extracted by said extracting unit.