docs: function handleStepsArray(that, stageId, steps, method) {
          var i, len;
          i = -1, len = steps.length;
          // Missing steps are added with default callback (if string),
          // or as they are, if object.
          for ( ; ++i < len ; ) {
              if ('object' === typeof steps[i]) {
                  // Throw error if step.id is not unique.
                  that.addStep(steps[i]);
                  // Substitute with its id.
                  steps[i] = steps[i].id;
              }
              else if ('string' === typeof steps[i]) {
                  if (!that.steps[steps[i]]) {
                      // Create a step with a default cb (will be substituted).
                      // Note: default callback and default step are two
                      // different things.
                      that.addStep({
                          id: steps[i],
                          cb: that.getDefaultCb()
                      });
                  }
              }
              else {
                  throw new TypeError('Stager.' + method + ': stage ' +
                                      stageId  + ': each item in the steps ' +
                                      ' array must be string or object. Found: ' +
                                      steps[i]);
              }
          }
      }