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(emacs-lisp-intro.info)cons

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`cons'
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   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' puts a new element at the beginning of
a list; it attaches or pushes elements onto the list.

Build a list

length

How to find the length of a list.