CLASSIFICATION AND SETS Classification o f some kind o c c u r s in eel but the simplest computer programs, but Its use is often Implicit and unrecognlzed. Making classification ezplicit can often lead t o clearer and m o r e efficient programs. T h e p r e s e n t n o t e illustrates the use of certain general tFpes of classification. In ~xzrti~Zar, it presents a treatment of s e t s that is simpler and more usezbZe than that provided b~ the conventional set notation of mathematics. The reader is encouraged to exploit the executability of the AP[ ezpresaiona by entering them on a suitable c o m ~ t e r sFstem and e z p e r imenting with them and with variations that maw suggest themselves. W e w i l l u s e direct definition and various new functions and operators defined in the D i o t i o n o ~ of APL [I], X~t will also provide alternative e ~ r e s s i o n s executable in I S O ~ t a ~ d a r d ~
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