function clsXY(w, h) {this.x = w; this.y = h;}
clsXY.prototype.center = function(another) { return new clsXY((this.x - another.x) / 2, (this.y - another.y) / 2); }
clsXY.prototype.show = function() {alert("x: " + this.x + "; y: " + this.y); return this; }
clsXY.prototype.add = function(another) { return new clsXY(this.x + another.x, this.y + another.y); }
clsXY.prototype.toString = function() {return this.x + ":" + this.y;}