Rect = {x = 0, y = 0, width = 10, height = 20}

Rect.__index = Rect

function Rect:new (o)
  setmetatable(o, self)
  return o
end

function Rect:area ()
  return self.width * self.height
end

r1 = Rect:new {width = 40, height = 60}
print(r1:area())
r2 = Rect:new {}
print(r2:area())
r1.width = 3
print(r1:area())
print(r2:area())