Consider this frink program:

x = new interval[-1,1]
y = new interval[-1,1]
println[x-y]
println[x-x]

The results are:

[-2, 2]
[-2, 2]

The first one makes sense: subtracting a number between -1 and 1 from
another number between -1 and 1 yields a number between -2 and 2.

Is the second one correct? Subtracting a number between -1 and 1 from
itself should yield 0, no? Thus, the second answer should be [0,0],
no?

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