> It's unfortunate that only some languages support them. yes, you need runtime support for this feature. If it is impossible to change the runtime than it ca

> It's unfortunate that only some languages support them. yes, you need runtime support for this feature. If it is impossible to change the runtime than it can't be done for the language; now the runtime may have problem to work with a very limited stack for example, or the runtime is big and difficult to change (like the JVM), or it has long library functions that cannot yield to other co threads. However C/C++ or other languages that do not sit on top of a big runtime dont have that problem.