Purpose: Definition of set functionality for an official extension.
----
   AK (1999, Mar 17/18):   Had to revamp all the Set pages due to the removal of clutter from various scripts and the introduction of a new way of doing things, which had to be included into the timings, of course. This new way of operation uses the local namespace of a procedure as an implicit array, fast to retrieve, and fast to set. This principle was fastest among all currently tested functions and implementation variants. Its disadvantage is however that no set must contain the names of the internal variables required in the computation itself, or incorrect results will occur. This should be avoidable, if documented properly, but is nevertheless an incitement to switch to a C implementation in the future.

----

   * Determine membership of an element in a list.
   * Compute the union of 2 sets represented by 2 lists.
   * Compute the intersection of 2 sets represented by 2 lists.
   * Compute the symmetric difference of 2 sets represented by 2 lists ('''symmetric difference''' being the set containing those elements which are in one but not the other, the inverse of intersection.).
   * Compute the onesided difference of 2 sets represented by 2 lists ('''onesided difference''' being the set containing those elements which are in the first but not the second.).
   * Check for an empty list, eh, set.

   Sometimes intersection and both onesided differences are
   required. A procedure computing them in parallel should be more
   efficient than doing it separately.

   * A procedure to compute intersection and both onesided differences.

   AK (Mar 20, 1999): The timing of the possible implementations
   proved me wrong, this procedure is not necessary.

----

   Remark:   The first implementation should use lists to represent sets, as they can be transfered by value. A C implementation OTOH should define a new Tcl_Obj-Type.

----

   Name of the extension:   SetOps

----

Implementation:
   Namespace:   ::setops

General features:
   * Sets are represented on the tcl-level by lists. A C implementation does not exist yet, but may employ hash-tables to gain performance.

Functions:

   * [SetOps, Create]                 ::setops::create elements
   * [SetOps, Contains]               ::setops::contains set element
   * [SetOps, Union]                  ::setops::union set1 set2...
   * [SetOps, Intersection]           ::setops::intersection set1 set2...
   * [SetOps, Difference]             ::setops::diff set1 set2
   * [SetOps, Symmetrical difference] ::setops::symdiff set1 set2
   * [SetOps, Emptiness]              ::setops::empty set
   * [SetOps, Intersect3]             ::setops::intersect3 set1 set2

Code:

   * [SetOps, Code, 7.6] -- no inline documentation yet.
   * [SetOps, Code, 8.x] -- no inline documentation yet.

Testcases:

   * '''not yet'''

----
Discussion area.

LV: Are there other list oriented languages from whom we can 'borrow'
set functionality ?  Perl's idea of adopting concepts found in other
languages, done in the spirit of perl, is one that could really work
in Tcl as well.         Mar 20, 1999

----
-- AK