'Substituion on the Vector.
Examples
>>> from sympy.physics.vector import ReferenceFrame
>>> from sympy import Symbol
>>> N = ReferenceFrame(\'N\')
>>> s = Symbol(\'s\')
>>> a = N.x * s
>>> a.subs({s: 2})
2*N.x'

def subs(self, *args, **kwargs):

    d = {}
    for v in self.args:
        d[v[1]] = v[0].subs(*args, **kwargs)
    return Vector(d)
