code: function(){
  
              parent.recalc.call(this);
              // moment of inertia
              var b
                  ,moi = 0
                  ;
  
              for ( var i = 0, l = this.children.length; i < l; i++ ) {
                  b = this.children[ i ];
                  b.recalc();
                  // parallel axis theorem
                  moi += b.moi + b.mass * b.state.pos.normSq();
              }
  
              this.moi = moi;
              return this;
          }
docs: extended