'Returns the exponent of the magnitude of the operand\'s MSD.
The result is the integer which is the exponent of the magnitude
of the most significant digit of the operand (as though the
operand were truncated to a single digit while maintaining the
value of that digit and without limiting the resulting exponent).
>>> ExtendedContext.logb(Decimal(\'250\'))
Decimal(\'2\')
>>> ExtendedContext.logb(Decimal(\'2.50\'))
Decimal(\'0\')
>>> ExtendedContext.logb(Decimal(\'0.03\'))
Decimal(\'-2\')
>>> ExtendedContext.logb(Decimal(\'0\'))
Decimal(\'-Infinity\')
>>> ExtendedContext.logb(1)
Decimal(\'0\')
>>> ExtendedContext.logb(10)
Decimal(\'1\')
>>> ExtendedContext.logb(100)
Decimal(\'2\')'

def logb(self, a):

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