An Extended Notion of Backward Compatibility

This version adds a clarification distinguishing between theoretical existence and practical realizability of transformation algorithms. This note proposes an extended notion of backward compatibility in programming languages and software systems. Unlike the traditional approach, which treats backward compatibility as the preservation of identical interpretation of constructs and data across versions, we suggest understanding it as the existence of a formal and guaranteed transformation that ensures semantic equivalence between versions. It is argued that such an extension more accurately reflects the actual practice of language and system evolution and helps to remove a number of formal constraints that have no essential practical significance.