docs: function(v) {
          if (v instanceof Vec3) {
              return new Vec3(this.x + v.x, this.y + v.y, this.z + v.z);
          }
          else if(v instanceof Vec2){
              return new Vec3(this.x + v.x, this.y + v.y, this.z);
          }
          else if(typeof v === 'number') {
              return new Vec3(this.x + v, this.y + v, this.z + v);
          }
          else {
              throw new TypeError('Invalid argument', v);
          }
      }