'Returns the same Decimal object.
As we do not have different encodings for the same number, the
received object already is in its canonical form.
>>> ExtendedContext.canonical(Decimal(\'2.50\'))
Decimal(\'2.50\')'

def canonical(self, a):

    if (not isinstance(a, Decimal)):
        raise TypeError('canonical    requires    a    Decimal    as    an    argument.')
    return a.canonical()
