Rules for Extending

Any modification to an existing API MUST avoid breaking changes and MUST maintain backward compatibility.

In particular, any change to an API MUST follow the following Rules for Extending:

1. You MUST NOT take anything away (related: Minimal Surface Principle

   , Robustness Principle)

2. You MUST NOT change processing rules

3. You MUST NOT make optional things required

4. Anything you add MUST be optional (related Robustness Principle)

NOTE: These rules cover also renaming and change to identifiers (URIs). Names and identifiers should be stable over the time including their semantics.