kill(all);

arbitrary parameters:
. exx,eyy,gxy
. Rtx,Rty,RrOz

in-plane rigid body motions,
for arbitrary
Rtx, Rty, RrOz

u_rbm: Rtx-RrOz*y;
v_rbm: Rty+RrOz*x;
psi_rbm:RrOz;

membrane terms

u  :u_rbm+exx*x+(  l)*gxy*y;
v  :v_rbm+      (1-l)*gxy*x+eyy*y;
psi:psi_rbm;

diff(u,x);

diff(v,y);

diff(u,y)+diff(v,x)$
fullratsimp(%);

TODO for out-of-plane rigid body motions, and curvatures,
with arbitrary
- kxx,kyy,kxy
- Rtz, RrOx, RrOy