You can sort of add new keywords backwards-compatibly using a trick called "contextual keywords": you require that they be placed in a syntactic position in whi

You can sort of add new keywords backwards-compatibly using a trick called "contextual keywords": you require that they be placed in a syntactic position in which no identifier could legally go, and you maintain them as legal identifiers for compatibility. C++ used this trick to introduce "final" and "override" by moving them before the opening "{".