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To create a collection within a collection in a Flutter document, you can follow these steps:

1. Create a reference to the parent document:

final DocumentReference parentDocRef = Firestore.instance.collection('parentCollection').document('parentDocId');

1. Use the parent document reference to create a reference to the child collection:

final CollectionReference childCollectionRef = parentDocRef.collection('childCollection');

1. Add a new document in the child collection:

childCollectionRef.add({'field1': 'value1', 'field2': 'value2'});

1. You can also retrieve documents from the child collection:

childCollectionRef.getDocuments().then((QuerySnapshot snapshot) {
  snapshot.documents.forEach((DocumentSnapshot document) {
    // process document data
  });
});