code: function isRefObject (thing) {
    if (thing && typeof thing === 'object' && !Array.isArray(thing)) {
      const keys = Object.keys(thing)
      return keys.length === 1 && keys[0] === '$ref' && typeof thing.$ref === 'string'
    }
    return false
  }
docs: Determines if object is a $ref object. That is { $ref: <something> }
  @param thing object to test
  @returns {boolean} true if passes the test. false otherwise.
  @private