combines two values into a pair
(cons car cdr)
cdr is an ordered sequence, such as a list or vector, the result is a new list or vector with the
car value prepended to the original. With an unordered sequence, such as an object array, there is no guarantee regarding position. If
cdr is not a sequence, then a new cons cell will be constructed.
The name cons is a vestige of when Lisp implementations constructed new lists or cells by pairing a car (contents of the address part of register) with a cdr (contents of the decrement part of register).
(define x '(3 4 5 6)) (define y (cons 2 x)) (cons 1 y)
This example will return (1 2 3 4 5 6).