资料来源 : Free On-Line Dictionary of Computing

DeMorgan's theorem
     
         A logical {theorem} which states that the {complement}
        of a {conjunction} is the {disjunction} of the complements or
        vice versa.  In symbols:
     
        	not (x and y) = (not x) or (not y)
        	not (x or y)  = (not x) and (not y)
     
        E.g. if it is not the case that I am tall and thin then I am
        either short or fat (or both).  The theorem can be extended to
        combinations of more than two terms in the obvious way.
     
        The same laws also apply to sets, replacing logical complement
        with set complement, conjunction ("and") with set
        intersection, and disjunction ("or") with set union.
     
        A ({C}) programmer might use this to re-write
     
        	if (!foo && !bar) ...
        as
        	if (!(foo || bar)) ...
     
        thus saving one operator application (though an {optimising
        compiler} should do the same, leaving the programmer free to
        use whichever form seemed clearest).
     
        (1995-12-14)