PROCEDURE DO_TOGGLE;
   BEGIN
      Hide_Mouse;
      toggle_inverse(Black,data_row,data_col);
      Show_Mouse
   END; { DO_TOGGLE }

PROCEDURE DO_DRAW;
   BEGIN
      Hide_Mouse;
      draw_cell(data_row,data_col,FALSE);
      Show_Mouse;
      write_cell_name
   END; { DO_DRAW }
   
FUNCTION ASSIGN_IF_POSSIBLE : BOOLEAN;
   VAR ptr : CellPtr;
   BEGIN
       assign_if_possible := FALSE;
       IF NOT null_input THEN BEGIN
          ptr := assign(temp);
          IF ptr <> NIL THEN
             IF ptr^.status <> Full THEN BEGIN
                cell_on_screen(1,data_row,data_col,TRUE);
                write_cell_name;
                GOTO 2 { label in evaluate_input in opus.pas; so we }
             END       { stay in the cell where the error occurred  }
             ELSE
                assign_if_possible := TRUE
       END
   END; { ASSIGN_IF_POSSIBLE }

PROCEDURE DO_SINCE_AT_BOUNDS ( msg_num : INTEGER; changed : BOOLEAN );
   { was at edge of screen and move off-screen }
   BEGIN
       IF changed THEN
          cell_on_screen(1,data_row,data_col,TRUE);
       simulate_message(WM_Arrowed,act_hdl,msg_num)
   END;
PROCEDURE ROW_CONSTANT_DRAW ( direction : INTEGER; changed : BOOLEAN );
   { moved a column, within screen area }
   BEGIN
       IF changed THEN
          cell_on_screen(1,data_row,data_col,TRUE);
       do_toggle;
       data_col := data_col+direction;
       do_draw
   END;
PROCEDURE COL_CONSTANT_DRAW ( direction : INTEGER; changed : BOOLEAN );
   { moved a row, within screen area }
   BEGIN
       IF changed THEN
          cell_on_screen(1,data_row,data_col,TRUE);
       do_toggle;
       data_row := data_row+direction;
       do_draw
   END;

PROCEDURE LEFT_ARROW;
   VAR
      did_assign : BOOLEAN;
   BEGIN
       did_assign := assign_if_possible;
       IF data_col = start_col THEN
          do_since_at_bounds(6,did_assign)
       ELSE
          row_constant_draw(-1,did_assign)
   END; (* LEFT ARROW *)

PROCEDURE RIGHT_ARROW;
   VAR
       did_assign : BOOLEAN;
   BEGIN
       did_assign := assign_if_possible;
       IF data_col = finish_col THEN
          do_since_at_bounds(7,did_assign)
       ELSE
          row_constant_draw(1,did_assign)
   END; (* RIGHT_ARROW *)

PROCEDURE UP_ARROW;
   VAR
      did_assign : BOOLEAN;
   BEGIN
       did_assign := assign_if_possible;
       IF data_row = start_row THEN
          do_since_at_bounds(2,did_assign)
       ELSE
          col_constant_draw(-1,did_assign)
   END; (* UP_ARROW *)

PROCEDURE DOWN_ARROW;
   VAR
      did_assign : BOOLEAN;
   BEGIN
       did_assign := assign_if_possible;
       IF data_row = finish_row THEN
          do_since_at_bounds(3,did_assign)
       ELSE
          col_constant_draw(1,did_assign)
   END; (* DOWN_ARROW *)

FUNCTION ON_SCREEN ( what : INTEGER; rows : BOOLEAN ) : BOOLEAN;
   BEGIN
      on_screen := FALSE;
      IF rows THEN
         IF (what >= start_row) AND (what <= finish_row) THEN
            on_screen := TRUE
         ELSE
      ELSE IF (what >= start_col) AND (what <= finish_col) THEN
         on_screen := TRUE
   END; { ON_SCREEN }
            
PROCEDURE DO_AUTO_CURSOR;
   VAR did_assign : BOOLEAN;
   BEGIN
       did_assign := assign_if_possible;
       IF did_assign THEN
          cell_on_screen(1,data_row,data_col,TRUE);
       IF cursor_direction = CursorRight THEN
          IF data_col = b_e_col THEN
             IF data_row <> b_e_row THEN
                IF data_row = finish_row THEN
                   IF on_screen(b_s_col,FALSE) THEN BEGIN
                      simulate_message(WM_Arrowed,act_hdl,3);
                      do_toggle;
                      data_col := b_s_col;
                      do_draw
                   END
                   ELSE BEGIN
                      data_row := data_row+1;
                      data_col := b_s_col;
                      start_row := data_row;
                      start_col := data_col;
                      Send_Redraw(FALSE,0,0,screen_width,screen_height)
                   END
                ELSE IF on_screen(b_s_col,FALSE) THEN BEGIN
                   do_toggle;
                   data_row := data_row+1;
                   data_col := b_s_col;
                   do_draw
                END
                ELSE BEGIN
                   data_row := data_row+1;
                   data_col := b_s_col;
                   start_row := data_row;
                   start_col := data_col;
                   Send_Redraw(FALSE,0,0,screen_width,screen_height)
                END
             ELSE
          ELSE IF data_col = finish_col THEN
             simulate_message(WM_Arrowed,act_hdl,7)
          ELSE BEGIN
             do_toggle;
             data_col := data_col+1;
             do_draw
          END      
       { direction was CursorDown }      
       ELSE IF data_row = b_e_row THEN { last marked row } 
          IF data_col <> b_e_col THEN { is it last marked cell? }
             IF data_col = finish_col THEN
                IF on_screen(b_s_row,TRUE) THEN BEGIN
                   simulate_message(WM_Arrowed,act_hdl,7);
                   do_toggle; { the blit assumed that cell on right is new }
                   data_row := b_s_row; { current cell but it is not! }
                   do_draw
                END
                ELSE BEGIN
                   data_row := b_s_row;
                   data_col := data_col+1;
                   start_row := data_row;
                   start_col := data_col;
                   Send_Redraw(FALSE,0,0,screen_width,screen_height)
                END
             ELSE IF on_screen(b_s_row,TRUE) THEN BEGIN
                do_toggle;
                data_row := b_s_row;
                data_col := data_col+1;
                do_draw
             END
             ELSE BEGIN
                data_row := b_s_row;
                data_col := data_col+1;
                start_row := data_row;
                start_col := data_col;
                Send_Redraw(FALSE,0,0,screen_width,screen_height)
             END
          ELSE   
       ELSE IF data_row = finish_row THEN
          simulate_message(WM_Arrowed,act_hdl,3)
       ELSE BEGIN
          do_toggle;
          data_row := data_row+1;
          do_draw
       END
   END; { DO_AUTO_CURSOR }