Section concat.
Context X `{registered X}.
Fixpoint rev_concat acc (xs : list (list X)) :=
match xs with
[] => acc
| x::xs => rev_concat (rev_append x acc) xs
end.
Global Instance term_rev_concat : computableTime' rev_concat _ := projT2 _term_rev_concat.

Lemma _term_concat :
{ time : UpToC (fun l => sumn (map (@length _) l) + length l + 1) &
computableTime' (@concat X) (fun l _ => (time l,tt))}.