-- rectangle prototype:
rectangle_proto= {species="rectangle"}
function rectangle_proto:area() return self.width * self.height end

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

-- rectangle usage:
rectangle= new_rectangle(3,4)
print(rectangle:area())  --> 12
print(rectangle.species) --> rectangle


-- circle prototype:
circle_proto= {species="circle"}
function circle_proto:area() return self.radius^2 * math.pi end

-- circle constructor:
function new_circle(radius)
    local circle= {radius= radius}
    setmetatable(circle, {__index=circle_proto})
    return circle
end

circle= new_circle(1)
print(circle:area())  --> 3.14
print(circle.species) --> circle