Many Schemes support a variety of commands in their REPLs to do various things you might want to do at a REPL, such as change its state in some way.
Command prefixed by comma (that is, wrapped in (unquote ...): Chicken, Gambit, Gauche, Guile, MIT, Otus, Owl Lisp,
Racket, Rep, Scheme 9, Scheme 48, STklos.
Command is a control character: MIT.
Command prefixed by @: Chibi (with -R option).