Sequences
A sequence can be thought of as a mapping from
{0, 1, ..., n-1} to a set of range elements.
A string is a sequence of characters.
A list is a particular form of sequence of lisp objects.
(typep "abc" 'sequence) ; => T.
(typep '(a b c) 'sequence) ; => T.
(concatenate 'string "abc" "def"); => "abcdef"
(concatenate 'list '(a b) '(c d)); =>(a b c d)