'Return specfic state by index.
Parameters
index : index of state to be returned
Examples
>>> from sympy.physics.quantum.state import Ket
>>> from sympy.physics.quantum.density import Density
>>> d = Density([Ket(0), 0.5], [Ket(1),0.5])
>>> d.states()[1]
|1>'

def get_state(self, index):

    state = self.args[index][0]
    return state
