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Migration policy from v3 to v4

I have a question regarding the code in the migration policy file.
Why there is a function inside a func. nothing wrong but it’s not very common practice.

I am wondering if there is reason behind it. Please help me understand.
Code snippet below:

class AttachmentToImageAttachmentMigrationPolicyV3toV4: NSEntityMigrationPolicy {
  override **func** createDestinationInstances(forSource sInstance: NSManagedObject, in mapping: NSEntityMapping, manager: NSMigrationManager) throws {
...
    **func** traversePropertyMappings(block: (NSPropertyMapping, String) -> Void) throws

@alokc83 Please check out the updated version of the book:

https://store.raywenderlich.com/products/core-data-by-tutorials

I hope it helps!