local M= {}
local private= {} -- holds private data of all objecs in subtables

-- Class Point: --

local Point_proto= {} -- Class prototype
function Point_proto:set(x,y)
    local priv= private[self]
    priv.x= x
    priv.y= y
end
function Point_proto:get()
    local priv= private[self]
    return priv.x, priv.y
end
function Point_proto:tostring()
    local priv= private[self]
    return "["..priv.x.."|"..priv.y.."]"
end
Point_proto.__index   = Point_proto
Point_proto.__newindex= function() assert(false,"object is write-protected") end

setmetatable(Point_proto, { -- ensure read protection for Point and all childs
    __index= function() assert(false, "object is read-protected") end
})

function M.new_Point(x,y) -- constructor
    local instance= {}
    setmetatable(instance, Point_proto)
    private[instance]= {} -- create private part of object
    instance:set(x,y)
    return instance
end

return M