cons-stream

(cons-stream a b)

Syntax, not a procedure: builds a stream whose first element is the value of a and whose rest is b, which is not evaluated until the stream's cdr is first asked for, and then only once. A stream is '() or a pair whose cdr is a promise, so infinite streams are ordinary. A Structure and Interpretation of Computer Programs (SICP)/MIT name, not R7RS: no library exports it, so it is visible only to a file with no import and at the REPL.