code: 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);
      }
docs: #### checkStepValidity
  
  Returns whether given step is valid
  
  Checks for syntactic validity of the step object. Does not validate
  whether the name is unique, etc.
  
  @param {object} step The step object
  @param {string} method The name of the method calling the validation
  
  @see Stager.addStep
  @see checkStageStepId
  
  @api private