[issue1682] Move Demo/classes/Rat.py to Lib/rational.py and fix it up.

[Facundo Batista]

Facundo Batista added the comment:

I'm +0 to make Decimal(Rational(x,y)) available.

I'd make it something like:

Decimal(R) == Decimal(R.numerator) / Decimal(R.denominator)

Note, as Raymond says, that this division implies the use of the
context. So the following will NOT be true:

R == Rational(Decimal(R))

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