code: function (meta, fn) {
      if (!_.isObject(meta) || meta.version === undefined || meta.version === null || !_.isNumber(meta.version)) {
        throw new Error('Please pass in a version');
      }
      if (!_.isFunction(fn)) {
        throw new Error('Please pass in a function');
      }
  
      this.snapshotConversionRegistrations.push({ meta: meta, fn: fn });
      return this;
    }
docs: Defines a new conversion function for older snapshot versions.
  @param {Object}   meta Meta infos like: { version: 10 }
  @param {Function} fn   Function containing the conversion rule
  `function(snapshotData, aggregateModel){}`
  @returns {Aggregate}