code: function getDefaultState() {
      return { depth: 0, seen: new WeakMap(), top: true, combine: false, allowRefSiblings: false };
  }
docs: functions to walk an OpenAPI schema object and traverse all subschemas
  calling a callback function on each one
   
  obtains the default starting state for the `state` object used
  by walkSchema
  @return the state object suitable for use in walkSchema