docs: function checkStepValidity(step, method) {
          if (step === null || 'object' !== typeof step) {
              throw new TypeError('Stager.' + method + ': step must be ' +
                                  'object. Found: ' + step);
          }
          if ('function' !== typeof step.cb) {
              throw new TypeError('Stager.' + method + ': step.cb must be ' +
                                  'function. Found: ' + step.cb);
          }
          checkStageStepId(method, 'step', step.id);
      }