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(cl)Lists as Sets

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Lists as Sets
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These functions perform operations on lists which represent sets of
elements.

 - Function: member* item list &key :test :test-not :key
     This function searches LIST for an element matching ITEM.  If a
     match is found, it returns the cons cell whose `car' was the
     matching element.  Otherwise, it returns `nil'.  Elements are
     compared by `eql' by default; you can use the `:test',
     `:test-not', and `:key' arguments to modify this behavior.  Note:
     Sequences.

     Note that this function's name is suffixed by `*' to avoid the
     incompatible `member' function defined in Emacs.  (That function
     uses `equal' for comparisons; it is equivalent to `(member* ITEM
     LIST :test 'equal)'.)

   The `member-if' and `member-if-not' functions analogously search for
elements which satisfy a given predicate.

 - Function: tailp sublist list
     This function returns `t' if SUBLIST is a sublist of LIST, i.e.,
     if SUBLIST is `eql' to LIST or to any of its `cdr's.

 - Function: adjoin item list &key :test :test-not :key
     This function conses ITEM onto the front of LIST, like `(cons ITEM
     LIST)', but only if ITEM is not already present on the list (as
     determined by `member*').  If a `:key' argument is specified, it
     is applied to ITEM as well as to the elements of LIST during the
     search, on the reasoning that ITEM is "about" to become part of
     the list.

 - Function: union list1 list2 &key :test :test-not :key
     This function combines two lists which represent sets of items,
     returning a list that represents the union of those two sets.  The
     result list will contain all items which appear in LIST1 or LIST2,
     and no others.  If an item appears in both LIST1 and LIST2 it will
     be copied only once.  If an item is duplicated in LIST1 or LIST2,
     it is undefined whether or not that duplication will survive in the
     result list.  The order of elements in the result list is also
     undefined.

 - Function: nunion list1 list2 &key :test :test-not :key
     This is a destructive version of `union'; rather than copying, it
     tries to reuse the storage of the argument lists if possible.

 - Function: intersection list1 list2 &key :test :test-not :key
     This function computes the intersection of the sets represented by
     LIST1 and LIST2.  It returns the list of items which appear in
     both LIST1 and LIST2.

 - Function: nintersection list1 list2 &key :test :test-not :key
     This is a destructive version of `intersection'.  It tries to
     reuse storage of LIST1 rather than copying.  It does _not_ reuse
     the storage of LIST2.

 - Function: set-difference list1 list2 &key :test :test-not :key
     This function computes the "set difference" of LIST1 and LIST2,
     i.e., the set of elements that appear in LIST1 but _not_ in LIST2.

 - Function: nset-difference list1 list2 &key :test :test-not :key
     This is a destructive `set-difference', which will try to reuse
     LIST1 if possible.

 - Function: set-exclusive-or list1 list2 &key :test :test-not :key
     This function computes the "set exclusive or" of LIST1 and LIST2,
     i.e., the set of elements that appear in exactly one of LIST1 and
     LIST2.

 - Function: nset-exclusive-or list1 list2 &key :test :test-not :key
     This is a destructive `set-exclusive-or', which will try to reuse
     LIST1 and LIST2 if possible.

 - Function: subsetp list1 list2 &key :test :test-not :key
     This function checks whether LIST1 represents a subset of LIST2,
     i.e., whether every element of LIST1 also appears in LIST2.