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(cl)Sorting Sequences

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Sorting Sequences
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 - Function: sort* seq predicate &key :key
     This function sorts SEQ into increasing order as determined by
     using PREDICATE to compare pairs of elements.  PREDICATE should
     return true (non-`nil') if and only if its first argument is less
     than (not equal to) its second argument.  For example, `<' and
     `string-lessp' are suitable predicate functions for sorting
     numbers and strings, respectively; `>' would sort numbers into
     decreasing rather than increasing order.

     This function differs from Emacs' built-in `sort' in that it can
     operate on any type of sequence, not just lists.  Also, it accepts
     a `:key' argument which is used to preprocess data fed to the
     PREDICATE function.  For example,

          (setq data (sort data 'string-lessp :key 'downcase))

     sorts DATA, a sequence of strings, into increasing alphabetical
     order without regard to case.  A `:key' function of `car' would be
     useful for sorting association lists.

     The `sort*' function is destructive; it sorts lists by actually
     rearranging the `cdr' pointers in suitable fashion.

 - Function: stable-sort seq predicate &key :key
     This function sorts SEQ "stably", meaning two elements which are
     equal in terms of PREDICATE are guaranteed not to be rearranged
     out of their original order by the sort.

     In practice, `sort*' and `stable-sort' are equivalent in Emacs
     Lisp because the underlying `sort' function is stable by default.
     However, this package reserves the right to use non-stable methods
     for `sort*' in the future.

 - Function: merge type seq1 seq2 predicate &key :key
     This function merges two sequences SEQ1 and SEQ2 by interleaving
     their elements.  The result sequence, of type TYPE (in the sense
     of `concatenate'), has length equal to the sum of the lengths of
     the two input sequences.  The sequences may be modified
     destructively.  Order of elements within SEQ1 and SEQ2 is
     preserved in the interleaving; elements of the two sequences are
     compared by PREDICATE (in the sense of `sort') and the lesser
     element goes first in the result.  When elements are equal, those
     from SEQ1 precede those from SEQ2 in the result.  Thus, if SEQ1
     and SEQ2 are both sorted according to PREDICATE, then the result
     will be a merged sequence which is (stably) sorted according to
     PREDICATE.