How do I substitute vorticity components for velocity component derivatives within the Navier-Stokes equations in mupad or symbolic editor?

I am working on a form of the Navier-Stokes equations and am converting them to the vorticity-velocity form by taking the curl the momentum equations. Having taken the curl in mupad successfully I would now like to substitute the velocity component derivatives for the vorticity vector components. For example within the long equation i would like to find the combination [d(Uz)/dy-d(Uy)/dz] and substitute Wx in its place. Is there a way to do this in mupad or will it have to be done manually?

I tried using subsex( Equation, [d(Uz)/dy-d(Uy)/dz=Wx, d(Ux)/dz-d(Uz)/dx=Wy, d(Uy)/dx-d(Ux)/dy=Wz] ). It did nothing, presumably because the items in the larger equation are not grouped that way...

Below are my mupad inputs:

reset()

ux:=Ux(x,y,z,t)

uy:=Uy(x,y,z,t)

uz:=Uz(x,y,z,t)

[Prasad Mendu]

As you mentioned, it could be possible that the equation was not grouped to be suitable for using "subsex" as desired. It might be useful to look at the documentation about "subs" to see if that fits your use-case.

http://www.mathworks.com/help/symbolic/subs.html