elseif lines.current.pending.mode == 'arc' then
        local mx,my = coord(x-16), coord(y-lines.current.y)
        if mx >= 0 and mx < 256 and my >= 0 and my < lines.current.h then
          local center = lines.current.points[lines.current.pending.center]
          lines.current.pending.end_angle = angle_with_hint(center.x,center.y, mx,my, lines.current.pending.end_angle)
          table.insert(lines.current.shapes, lines.current.pending)
        end

  elseif shape.mode == 'arc' then
    local center = drawing.points[shape.center]
    love.graphics.arc('line', 'open', pixels(center.x)+left,pixels(center.y)+top, pixels(shape.radius), shape.start_angle, shape.end_angle, 360)
  else
    print(shape.mode)
    assert(false)

  elseif shape.mode == 'arc' then
    local center = drawing.points[shape.center]
    local mx,my = coord(love.mouse.getX()-16), coord(love.mouse.getY()-drawing.y)
    if mx < 0 or mx >= 256 or my < 0 or my >= drawing.h then
      return
    end
    shape.end_angle = angle_with_hint(center.x,center.y, mx,my, shape.end_angle)
    local cx,cy = pixels(center.x)+left, pixels(center.y)+top
    love.graphics.arc('line', 'open', cx,cy, pixels(shape.radius), shape.start_angle, shape.end_angle, 360)

  elseif shape.mode == 'arc' then
    local center = drawing.points[shape.center]
    local dist = math.dist(center.x,center.y, x,y)
    if dist < shape.radius*0.95 or dist > shape.radius*1.05 then
      return false
    end
    return angle_between(center.x,center.y, x,y, shape.start_angle,shape.end_angle)

function angle_between(x1,y1, x2,y2, s,e)
  local angle = math.angle(x1,y1, x2,y2)
--?   print(s,e, angle-math.pi*2, angle, angle+math.pi*2)
  if s > e then
    s,e = e,s
  end
  -- I'm not sure this is right or ideal..
  angle = angle-math.pi*2
  if s <= angle and angle <= e then
    return true
  end
  angle = angle+math.pi*2
  if s <= angle and angle <= e then
    return true
  end
  angle = angle+math.pi*2
  return s <= angle and angle <= e
end

  elseif love.mouse.isDown('1') and chord == 'a' and current_mode == 'circle' then
    local drawing = current_drawing()
    drawing.pending.mode = 'arc'
    local mx,my = coord(love.mouse.getX()-16), coord(love.mouse.getY()-drawing.y)
    local j = insert_point(drawing.points, mx,my)
    local center = drawing.points[drawing.pending.center]
    drawing.pending.radius = math.dist(center.x,center.y, mx,my)
    drawing.pending.start_angle = math.angle(center.x,center.y, mx,my)

end
function angle_with_hint(x1, y1, x2, y2, hint)
  local result = math.angle(x1,y1, x2,y2)
  if hint then
    -- Smooth the discontinuity where angle goes from positive to negative.
    -- The hint is a memory of which way we drew it last time.
    while result > hint+math.pi/10 do
      result = result-math.pi*2
    end
    while result < hint-math.pi/10 do
      result = result+math.pi*2
    end
  end
  return result
end
-- result is from -π/2 to 3π/2, approximately adding math.atan2 from Lua 5.3
-- (LÖVE is Lua 5.1)
function math.angle(x1,y1, x2,y2)
  local result = math.atan((y2-y1)/(x2-x1))
  if x2 < x1 then
    result = result+math.pi
  end
  return result