-- shape as base class:
shape_proto= {}
function shape_proto:tostring() return "["..self.width.."|"..self.height.."]" end

-- rectangle prototype inherits from shape:
rectangle_proto= {}
setmetatable(rectangle_proto, {
    __index=shape_proto
})

-- rectangle constructor:
function new_rectangle(width, height)
    rectangle= {width= width, height= height}
    setmetatable(rectangle, {
        __index=rectangle_proto,
        __tostring=rectangle_proto.tostring
    })
    return rectangle
end

-- rectangle usage:
rectangle= new_rectangle(3,4)
print(rectangle:tostring()) --> [3|4]
print(rectangle)            --> [3|4]