'Minus corresponds to unary prefix minus in Python.
The operation is evaluated using the same rules as subtract; the
operation minus(a) is calculated as subtract(\'0\', a) where the \'0\'
has the same exponent as the operand.
>>> ExtendedContext.minus(Decimal(\'1.3\'))
Decimal(\'-1.3\')
>>> ExtendedContext.minus(Decimal(\'-1.3\'))
Decimal(\'1.3\')
>>> ExtendedContext.minus(1)
Decimal(\'-1\')'

def minus(self, a):

    a = _convert_other(a, raiseit=True)
    return a.__neg__(context=self)
