docs: function addLoop(that, type, stage, loopFunc, positions) {
          var stageName;
  
          checkFinalized(that, type);
  
          stageName = handleStageParameter(that, stage, type);
  
          if ('function' !== typeof loopFunc) {
              throw new TypeError('Stager.' + type + ': loopFunc must be ' +
                                  'function. Found: ' + loopFunc);
          }
  
          positions = checkPositionsParameter(positions, type);
  
          addStageToCurrentBlock(that, {
              type: type,
              id: stageName,
              cb: loopFunc
          }, positions);
  
          // Must be done after addStageToCurrentBlock is called.
          addStepsToCurrentBlock(that, that.stages[stageName].steps);
          return that;
      }