`data_synth` unknown free variable bug

`data_synth` bugs on a simple function `fun ((a,b,c),d) => a`. There is something wrong with product association. ``` #check (HasFwdFDeriv Float (fun (((a,b,c),d) : (Float×Float×Float)×Float) => a) _) rewrite_by lsimp data_synth -domainDec -- unknown free variable '_fvar.19588' ```