Types Tutorial Exercise 9
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     1:  [-3 .. 3]       2:  [-3 .. 3]     2:  [0 .. 9]
         .               1:  [0 .. 9]      1:  [-9 .. 9]
                             .                 .

         [ 3 n .. 3 ]        RET 2 ^           TAB RET *

In the first case the result says, "if a number is between -3 and 3,
its square is between 0 and 9."  The second case says, "the product of
two numbers each between -3 and 3 is between -9 and 9."

An interval form is not a number; it is a symbol that can stand for
many different numbers.  Two identical-looking interval forms can
stand for different numbers.

The same issue arises when you try to square an error form.