'Performs the action of an annihilation operator.
Examples
>>> from sympy.physics.secondquant import BBra
>>> b = BBra([1, 2])
>>> b
FockStateBosonBra((1, 2))
>>> b.down(1)
FockStateBosonBra((1, 1))'

def down(self, i):

    i = int(i)
    new_occs = list(self.args[0])
    if (new_occs[i] == S.Zero):
        return S.Zero
    else:
        new_occs[i] = (new_occs[i] - S.One)
        return self.__class__(new_occs)
