ones complement

   A system used in some computers to represent negative
   numbers. To negate a number, each bit of the number is
   inverted (zeros are replaced with ones and vice versa).  This
   has the consequence that there are two reperesentations for
   zero, either all zeros or all ones.

   	...
   	000...00011 = +3
   	000...00010 = +2
   	000...00001 = +1
   	000...00000 = +0
   	111...11111 = -0
   	111...11110 = -1
   	111...11101 = -2
   	111...11100 = -3
   	...

   Naive logic for ones complement addition might easily conclude
   that -0 + 1 = +0.

   The twos complement avoids this by using all ones to
   represent -1.