docs: function checkStageValidity(stage, method) {
          if ('object' !== typeof stage) {
              throw new TypeError('Stager.' + method + ': stage must be ' +
                                  'object. Found: ' + stage);
          }
          if ((!stage.steps && !stage.cb) || (stage.steps && stage.cb)) {
              throw new TypeError('Stager.' + method + ': stage must have ' +
                                  'either a steps or a cb property');
          }
          if (J.isArray(stage.steps)) {
              if (!stage.steps.length) {
                  throw new Error('Stager.' + method + ': stage.steps cannot ' +
                                  'be empty');
              }
          }
          else if (stage.steps) {
              throw new TypeError('Stager.' + method + ': stage.steps must be ' +
                                  'array or undefined.  Found: ' + stage.steps);
          }
          checkStageStepId(method, 'stage', stage.id);
       }