docs: function _checkRefsInClasses (context) {
    for (const key of Object.keys(context.classes)) {
      const classNode = context.classes[key]
  
      if (_isImportNode(classNode)) {
        return null
      }
  
      const location = `Class.${classNode.label}.`
  
      let err = _checkPropertySpecs(context, classNode.propertySpecs)
      if (err) return `${location}${err}`
  
      if (classNode.subClassOf) {
        err = _checkClassReference(context, classNode.subClassOf)
        if (err) {
          return `${location}subClassOf ${err}`
        }
      }
  
      if (classNode.excludeParentProperties) {
        for (const parentProp of classNode.excludeParentProperties) {
          err = _checkPropertyReference(context, parentProp)
          if (err) return err
        }
      }
    }
  }