code: function create (entitySpec, id) {
    if (!_.isFunction(entitySpec)) {
      throw new Error(
        `Entity Factory '${id}' should be a function: ${JSON.stringify(
          entitySpec
        )}`
      )
    }
  
    if (entitySpec.length !== 2) {
      throw new Error(
        `Entity Factory '${id}' method should have an arity of 2, instead got: ${
          entitySpec.length
        }`
      )
    }
  
    return entitySpec
  }
docs: parse reducer
  @param  {Entity} entitySpec user defined entity
  @return {reducer}