docs: function () {
          var Compo = $component ({
                          position: $observableProperty (Vec2.zero),
                          init: function () {
                              this.positionChange (function (v) {
                                  $assertTypeMatches (v, Vec2)
                                  $assert (v.y, 42) }) } })
  
          var compo = new Compo ({ position: { x: 10, y: 42 }}) // supply POD value from constructor
          compo.position = { x: 20, y: 42 } }