docs: function( feature ) {
  			if ( this.disabled )
  				return true;
  
  			if ( !feature )
  				return true;
  
  			// Some features may want to register other features.
  			// E.g. a button may return a command bound to it.
  			if ( feature.toFeature )
  				feature = feature.toFeature( this.editor );
  
  			return !feature.requiredContent || this.check( feature.requiredContent );
  		}