This function organizes the contents of selection object selectionId and prepares for subsequent calls to tbl_selection_matchnext. A selection object contains a series of rectangles, each of which might cover an entire grid, one or more entire rows or columns, or simply a smaller area within a single grid. At different times, it may be advantageous to iteratively examine every cell in every rectangle; at others, a user might want to require ensure that only complete rows are selected, and iteratively process each row. tbl_selection_matchbegin supports all such possibilities by analyzing the selection object and organizing the rectangles it contains into the largest requested units. It returns '0' if the selection object contains a rectangle that can't be so organized, and '1' if the selection object contains rectangles that can be interpreted as the user indicates. In the latter case, calls to tbl_selection_matchnext can be used to retrieve the interpreted table objects.