At this point, the double conversions should really only be
implementation details of the KeypadValue class. Therefore,
the constructor-from double and conversion-operator-to
double of KeypadValue are made private. Instead, the
required functionality is provided by KeypadValue itself.
The internal implementation is still done using doubles.
However, this opens us up to the possibility of having
loss-free square root, inversion and division in the future.
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