the functionality in version N should be a strict superset of the functionality in version N-1. While this may seem obvious, large software projects tend to involve developers working in several parallel branches of the code, and there can be several concurrently supported releases of an API. It is therefore important that teams working in different parallel branches do not introduce incompatible features. There are several policy approaches to deal with this potential problem.
People working on same API on different branch can introduce incompatible behaviour or functionality of API.
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