test-and-set!

(test-and-set! cell)

Atomically tests and sets the car of the pair cell: when it is #f, sets it to #t and returns #f; otherwise leaves it alone and returns #t. The test and the set cannot be interleaved with another thread's, on every backend -- the primitive Structure and Interpretation of Computer Programs (SICP) builds a mutex on. A SICP/MIT name, not R7RS: no library exports it, so it is visible only to a file with no import and at the REPL.