** Emacs now provides a limited form of concurrency with Lisp threads.
Concurrency in Emacs Lisp is “mostly cooperative”, meaning that
Emacs will only switch execution between threads at well-defined
times: when Emacs waits for input, during blocking operations related
to threads (such as mutex locking), or when the current thread
explicitly yields. Global variables are shared among all threads, but
a ‘let’ binding is thread-local. Each thread also has its own current
buffer and its own match data.
See the chapter “Threads” in the ELisp manual for full documentation
of these facilities.