docs: function handleStepParameter(that, step, method) {
          var id;
          if ('object' === typeof step) {
              id = step.id;
              if (that.steps[id]) {
                  throw new Error('Stager.' + method + ': step is object, ' +
                                  'but a step with the same id already ' +
                                  'exists: ' + id);
              }
              // Add default callback, if missing.
              if (!step.cb) step.cb = that.getDefaultCallback();
          }
          else if ('string' === typeof step) {
              id = step;
              step = {
                  id: id,
                  cb: that.getDefaultCallback()
              };
          }
          else {
              throw new TypeError('Stager.' + method + ': step must be ' +
                                  'string or object. Found: ' + step);
          }
  
          // A new step is created if not found (performs validation).
          if (!that.steps[id]) that.addStep(step);
  
          return id;
      }