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cons
The cons function constructs lists; it is the inverse of
car and cdr. For example, cons can be used to make
a four element list from the three element list, (fir oak maple):
(cons 'pine '(fir oak maple))
After evaluating this list, you will see
(pine fir oak maple)
appear in the echo area. cons causes the creation of a new
list in which the element is followed by the elements of the original
list.
We often say that `cons puts a new element at the beginning of
a list; it attaches or pushes elements onto the list', but this
phrasing can be misleading, since cons does not change an
existing list, but creates a new one.
Like car and cdr, cons is non-destructive.