(lispref.info)Sequence Type

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Sequence Types

   A "sequence" is a Lisp object that represents an ordered set of
elements.  There are two kinds of sequence in Emacs Lisp, lists and
arrays.  Thus, an object of type list or of type array is also
considered a sequence.

   Arrays are further subdivided into strings and vectors.  Vectors can
hold elements of any type, but string elements must be characters in the
range from 0 to 255.  However, the characters in a string can have text
properties; vectors do not support text properties even when their
elements happen to be characters.

   Lists, strings and vectors are different, but they have important
similarities.  For example, all have a length L, and all have elements
which can be indexed from zero to L minus one.  Also, several
functions, called sequence functions, accept any kind of sequence.  For
example, the function `elt' can be used to extract an element of a
sequence, given its index.  Note: Sequences Arrays Vectors.

   It is impossible to read the same sequence twice, in the sense of
`eq' (Note: Equality Predicates.), since sequences are always created
anew upon reading.  There is one exception: the empty list `()' always
stands for the same object, `nil'.

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