code: function( integrator ){
  
              if ( integrator === undefined ){
                  return this._integrator;
              }
  
              // do nothing if already added
              if ( this._integrator === integrator ){
                  return this;
              }
  
              if ( this._integrator ){
  
                  this._integrator.setWorld( null );
  
                  this.emit( 'remove:integrator', {
                      integrator: this._integrator
                  });
              }
  
              if ( integrator ){
                  this._integrator = integrator;
                  this._integrator.setWorld( this );
  
                  this.emit( 'add:integrator', {
                      integrator: this._integrator
                  });
              }
  
              return this;
          }
docs: chainable
  Physics.world#integrator( [integrator] ) -> Integrator|this
  - integrator (Integrator): The integrator to set on the world
  + (Integrator): The currently set integrator if `integrator` not specified
  + (this): for chaining if `integrator` specified
  
  Get or Set the integrator