code: function( topic, fn, scope ){
  
              var listeners
                  ,listn
                  ;
  
              if ( !this._topics ){
                  // nothing subscribed
                  return this;
              }
  
              if ( topic === true ){
                  // purge all listeners
                  this._topics = {};
                  return this;
              }
  
              // check if we're subscribing to multiple topics
              // with an object
              if ( Physics.util.isObject( topic ) ){
  
                  for ( var t in topic ){
  
                      this.off( t, topic[ t ] );
                  }
  
                  return this;
              }
  
              listeners = this._topics[ topic ];
  
              if (!listeners){
                  return this;
              }
  
              if ( fn === true ){
                  // purge all listeners for topic
                  this._topics[ topic ] = [];
                  return this;
              }
  
              for ( var i = 0, l = listeners.length; i < l; i++ ){
  
                  listn = listeners[ i ];
  
                  if (
                      (listn._bindfn_ === fn || listn === fn) &&
                      ( (!scope) || listn._scope_ === scope) // check the scope too if specified
                  ){
                      listeners.splice( i, 1 );
                      break;
                  }
              }
  
              return this;
          }
docs: Physics.util.pubsub#off( topic, fn[, scope] ) -> this
  Physics.util.pubsub#off( topicCfg ) -> this
  - topic (String): topic The topic name. Specify `true` to remove all listeners for all topics
  - topicCfg (Object): A config with key/value pairs of `{ topic: callbackFn, ... }`
  - fn (Function): The original callback function. Specify `true` to remove all listeners for specified topic
  - scope (Object): The scope the callback was bound to. This is important if you are binding methods that come from object prototypes.
  
  Unsubscribe callback(s) from topic(s).