adjoin
builtin-function
(adjoin item seq)
Returns seq if seq contains item, otherwise returns a new list
with item as its new (first or last) element.
make-html > (adjoin 2 (list 1 2 3))
it: (1 2 3)
make-html > (adjoin 2 (list 1 3))
it: (2 1 3)
make-html > (adjoin 2 (vector 1 3))
it: (2 1 3)