code: function( thing ){
  
              var arr
                  ,i
                  ,l
                  ;
  
              if ( !thing ){
                  return false;
              }
  
              switch (thing.type){
  
                  case 'behavior':
                      arr = this._behaviors;
                  break; // end behavior
  
                  case 'integrator':
                  return ( this._integrator === thing );
                  // end integrator
  
                  case 'renderer':
                  return ( this._renderer === thing );
                  // end renderer
  
                  case 'body':
                      arr = this._bodies;
                  break; // end body
  
                  default:
                      throw 'Error: unknown type "'+ thing.type +'"';
                  // end default
              }
  
              // check array
              return (Physics.util.indexOf( arr, thing ) > -1);
          }
docs: chainable
  Physics.world#has( thing ) -> Boolean
  - thing (Object): The thing to test
  + (Boolean): `true` if thing is in the world, `false` otherwise.
  
  Determine if a thing has been added to world.