Flatten does not decompose

[Wout4]

No description provided.

[Wout4]

the change to get_or_make_var is a bit hacky, maybe there is a nicer way to achieve the same result

[Wout4]

Actually it was not neccesary, since the negation could be moved outside get_or_make_var. I did make a change to get_or_make_var that avoids unnecessary assignments in flat output

[tias]

if this is a cleanup, why did you close it unmerged? or it is superseded by another PR?

[Wout4]

I extracted the only useful code into the get_or_make_var pr
This addressed the fact that we now decompose in flatten, but that will be resolved with the push down negation and recursive decomposition transformations