'Return the Matrix converted into a one column matrix by stacking columns
Examples
>>> from sympy import Matrix
>>> m=Matrix([[1, 3], [2, 4]])
>>> m
Matrix([
[1, 3],
[2, 4]])
>>> m.vec()
Matrix([
[1],
[2],
[3],
[4]])
See Also
vech'

def vec(self):

    return self._eval_vec()
