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| ANSI Common Lisp 17 Sequences 17.3 Dictionary of Sequences
17.3.12 reverse, nreverse | Function
  Syntax: reverse sequence reversed-sequence nreverse sequence reversed-sequence Arguments and Values: sequence - a proper sequence. reversed-sequence - a sequence. Description: reverse and nreverse return a new sequence of the same kind as sequence, containing the same elements, but in reverse order. reverse and nreverse differ in that reverse always creates and returns a new sequence, whereas nreverse might modify and return the given sequence. reverse never modifies the given sequence. For reverse, if sequence is a vector, the result is a fresh simple array of rank one that has the same actual array element type as sequence. If sequence is a list, the result is a fresh list. For nreverse, if sequence is a vector, the result is a vector that has the same actual array element type as sequence. If sequence is a list, the result is a list. For nreverse, sequence might be destroyed and re-used to produce the result. The result might or might not be identical to sequence. Specifically, when sequence is a list, nreverse is permitted to setf any part, car or cdr, of any cons that is part of the list structure of sequence. When sequence is a vector, nreverse is permitted to re-order the elements of sequence in order to produce the resulting vector. Examples:
       (setq str "abc")  "abc"
       (reverse str)  "cba"
       str  "abc"
       (setq str (copy-seq str))  "abc"
       (nreverse str)  "cba"
       str  implementation-dependent
       (setq l (list 1 2 3))  (1 2 3)
       (nreverse l)  (3 2 1)
       l  implementation-dependent
Side Effects: nreverse might either create a new sequence, modify the argument sequence, or both. (reverse does not modify sequence.) Exceptional Situations: Should be prepared to signal an error of type type-error if sequence is not a proper sequence. Allegro CL Implementation Details: None.

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