28. The system of claim 27 wherein said best-effort parser performs a procedure comprising the steps of: building a schedule graph for determining the order of applying production rules; building multiple parse trees simultaneously by grouping tokens using production rules in determined orders; pruning useless parse trees by checking preference rules; and outputting multiple potential useful parse trees that maximally cover the tokens in the document.
28. The system of claim 27 wherein said best-effort parser performs a procedure comprising the steps of: building a schedule graph for determining the order of applying production rules; building multiple parse trees simultaneously by grouping tokens using production rules in determined orders; pruning useless parse trees by checking preference rules; and outputting multiple potential useful parse trees that maximally cover the tokens in the document. 29. The system of claim 28 , wherein the step of building a schedule graph comprises: adding dependency edges from head symbol to each component symbol for each production rule; adding restriction edges between two symbols for each preference rule; transforming restriction edges when graph is acyclic; and removing restriction edges if graph is still acyclic after said transformation.