+auto-fill every numeric hint, ready for removal as required, or the 'H'
+key to do the same but also to remove all obvious hints.
+
+Alternatively, use the cursor keys to move the mark around the grid.
+Pressing the return key toggles the mark (from a normal mark to a
+pencil mark), and typing a number in is entered in the square in the
+appropriate way; typing in a 0 or using the space bar will clear a
+filled square.