Before revision:  We explore how different basic approaches , i.e. the concurrency-centric approach taken by OT and the content-centric approach taken by CRDT, had resulted in different technical challenges , correctness and complexity issues , and solutions .
After revision:  We explore how different basic approaches , i.e. the concurrency-centric approach taken by OT and the content-centric approach taken by CRDT, had resulted in different challenges and consequential correctness and complexity issues , and solutions .