text: import NumberPair from './NumberPair.js'
  function Vector2(){
      NumberPair.apply(this,arguments)
  }
  Object.setPrototypeOf(Vector2,NumberPair)
  Object.setPrototypeOf(Vector2.prototype,NumberPair.prototype)
  // inner product
  Vector2.prototype.ip=function(v){
      return this.x*v.x+this.y*v.y
  }
  // length
  Object.defineProperty(Vector2.prototype,'len',{get(v){
      return this.ip(this)**.5
  }})
  Vector2.prototype.valueOf=function(){
      return this.ip(this)**.5
  }
  export default Vector2
  