Draw a horizontal "zapping" symbol on the canvas that shows
    that a graph is ripped in the middle.

    Parameter <fill_style> specifies the style for the zig-zag lines.
    PAT specifies the pattern with which the area is filled.
    The symbol is drawn in the rectangle (<x1>, <y1>) - (<x2>, <y2>).
    Each "zigzag" has the width <xsize>, height <ysize>.

def zap_horizontally(can, style, pat, x1, y1, x2, y2, xsize, ysize):
    """Draw a horizontal "zapping" symbol on the canvas that shows
    that a graph is ripped in the middle.

    Parameter <fill_style> specifies the style for the zig-zag lines.
    PAT specifies the pattern with which the area is filled.
    The symbol is drawn in the rectangle (<x1>, <y1>) - (<x2>, <y2>).
    Each "zigzag" has the width <xsize>, height <ysize>."""

    assert isinstance(style, line_style.T)
    assert isinstance(pat, fill_style.T)

    points = []
    points2 = []
    x = x1
    y = y1
    while x < x2:
        points.append((x, y))
        points2.append((x, y + (y2 - y1)))
        x += xsize
        if y == y1:
            y += ysize
        else:
            y -= ysize

    points2.reverse()
    _draw_zap(can, points, points2, style, pat)