ode and time discrete solver---week-end musing and learning

playing with comsol new features I stumble on solving an ode with the time discrete solver.
No matter what I do [ using bdf, prev operators as explained in the doc or in the exem-le pubished by Nicklas on this forum... I ALWAYS end with duplicate variable errors and some strange variable list in the equation vie mode
for exemple if I try to solve f1'=1, f1(0)=0

I will write in the ode node

bdf(f1,1)-11 as the equation

and put the proper initial condition

I get duplicate variable error names when I try to run
but then in the comsol generated equation some "strange variable are created f1t=bdf(f1,2) GENERATED by comsol and then
f1tt=bdf(bdf(f1,2),1)) again COMSOL generated .... and apparently wrong [ derivative shoud be first order bdf according to the doc...

Does somebody know the proper way to code global ODE for use of discrete time solver?

Or is there a bug/limitation I have not seen in the doc/release note...

I should say that I have no problem coding pde... ONLY global ODE...

I attach the small file corresponding to the little example above

Thanks
JF


EDIT I should add that the SAME coding as above BUT as a weak pde defined on a point on a geometry solve properly the equationbut then I need to export the solution in the glibal settting... complicated