3 is a perspective view of a cassette.FIG. 4 is a side view of the cassette.FIG. 5 is a diagram of the cassette as viewed from the above.FIGS. 6(a)-6(b) are elevation views of a pin and a pin receiver of the cassette, respectively.FIG. 7 is a sectional view of the cassette.FIG. 8 is a plan view of a slide resistance unit.FIG. 9 is an exploded perspective view of the slide resistance unit of FIG. 8