elseif chord == 'C-l' then
    for i,drawing in ipairs(lines) do
      if type(drawing) == 'table' then
        local x, y = love.mouse.getX(), love.mouse.getY()
        if y >= drawing.y and y < drawing.y + drawing.h and x >= 12 and x < 12+drawing.w then
          for j,shape in ipairs(drawing.shapes) do
            if on_freehand(love.mouse.getX(),love.mouse.getY(), shape) then
              convert_line(drawing,j,shape)
            end
          end
        end
      end
    end

end
function convert_line(drawing, i, shape)
  -- Perhaps we should do a more sophisticated "simple linear regression"
  -- here:
  --   https://en.wikipedia.org/wiki/Linear_regression#Simple_and_multiple_linear_regression
  -- But this works well enough for close-to-linear strokes.
  drawing.shapes[i] = {shape[1], shape[#shape]}