'Return Dirac conjugate (if self.rows == 4).
Examples
>>> from sympy import Matrix, I, eye
>>> m = Matrix((0, 1 + I, 2, 3))
>>> m.D
Matrix([[0, 1 - I, -2, -3]])
>>> m = (eye(4) + I*eye(4))
>>> m[0, 3] = 2
>>> m.D
Matrix([
[1 - I,     0,      0,      0],
[    0, 1 - I,      0,      0],
[    0,     0, -1 + I,      0],
[    2,     0,      0, -1 + I]])
If the matrix does not have 4 rows an AttributeError will be raised
because this property is only defined for matrices with 4 rows.
>>> Matrix(eye(2)).D
Traceback (most recent call last):
AttributeError: Matrix has no attribute D.
See Also
conjugate: By-element conjugation
H: Hermite conjugation'

@property
def D(self):

    from sympy.physics.matrices import mgamma
    if (self.rows != 4):
        raise AttributeError
    return (self.H * mgamma(0))
