'Return list of all states.
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()
(|0>, |1>)'

def states(self):

    return Tuple(*[arg[0] for arg in self.args])
