'Return probability of specific state by index.
Parameters
index : index of states whose probability is 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.probs()[1]
0.500000000000000'

def get_prob(self, index):

    prob = self.args[index][1]
    return prob
