20. A system according to claim 16 , wherein the grammar guesser is configured to automatically: for each element, assign at least one class to the element, wherein each of the at least one class is assigned to the element based at least in part based on the contents of the element; and for each row, assign a class to the row, wherein the class is assigned to the row based at least in part on the classes assigned to the elements of the row.
23. A system according to claim 20 , wherein the grammar guesser is configured to define, for each row, a trigger grammar and a filter grammar based at least in part on the class of the row, headings of the row and minimum and maximum values of the contents of elements of the row.