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Definition: z is a difference of x and y if x = y+z.
x - y := such z: x = z+y.
There is no z with 1=z+2, thus 1-2 is undefined.
forall x, y, z0, z1: (x = z0+y /\ x = z1+y) => z0 = z1.
forall x, y: x >= y => x= (x-y)+y.