Multi-integer design notes

This class simulate a two’s complement signed integer by using multiple unsigned native integers; this is portable.
The reason we require Word to be an unsigned native integer is that the bit-representation of signed integers is not fixed by the C++ standard; it could be ones’ complement, two’s complement, or sign-magnitude, or whatever.
By the C++ standard, all arithmetic on unsigned integers is modulo $2^n$ , where $n$ is the number of bits in the integer.
For example, (Word)-1 has all bits set to one, and $-x$ is such that $x + (-x) = 0$ (it’s the two’s complement negation).