length sequence => n
sequence---a proper sequence.
n---a non-negative integer.
Returns the number of elements in sequence.
If sequence is a vector with a fill pointer, the active length as specified by the fill pointer is returned.
(length "abc") => 3 (setq str (make-array '(3) :element-type 'character :initial-contents "abc" :fill-pointer t)) => "abc" (length str) => 3 (setf (fill-pointer str) 2) => 2 (length str) => 2
Should be prepared to signal an error of type type-error if sequence is not a proper sequence.