Will add the pertinent docstring.
'Scalar' is not a class on its own. Its just a sympy expression. I don't think adding an 'express' function to the sympy.core classes would make sense.
Consider finding gradient of sfield = 3 * R_x * R1_y where R and R1 have a certain relationship.
Therefore gradient would first do R.express(3*R_x*R1_y) and then do the necessary calculations on it.