Error: Cannot guess decreasing argument of fix. Coq - coq

Question

I have the following definition for terms :
Require Import Coq.Arith.Arith.
Require Import Coq.Lists.List.
Require Import Coq.Strings.String.
Import ListNotations.
Definition VarIndex:Type := nat.
Inductive Term : Type :=
|Var : VarIndex -> Term
|Comb: string -> (list Term) -> Term.
(*compare two list *)
Fixpoint beq_list {X:Type} (l l' :list X) (EqX :X -> X -> bool): bool :=
match l with
| [] => match l' with
| [] => true
| a => false
end
| (x::xs) =>
match l' with
|[] => false
|(y::ys) => if (EqX x y) then beq_list xs ys EqX else false
end
end.
Fixpoint length {X : Type} (l : list X) : nat :=
match l with
| nil => 0
| cons _ l' => S (length l')
end.
and a function beq_term to compare two terms define as follow :
Fixpoint beq_term (t1:Term) (t2:Term) : bool :=
match t1, t2 with
| Var i, Var j => beq_nat i j
| Var _, Comb _ _ => false
|Comb _ _, Var _ => false
|(Comb s1 ts1), Comb s2 ts2 => if(beq_nat (length ts1) (length ts2))
then beq_list ts1 ts2 beq_term
else false
end.
The definition of the function beq_term yields the error message:
Error: Cannot guess decreasing argument of fix.
So I am interested in how to convince Coq of the termination.

Answer 1

If you want to be able to use Coq's syntactic check in this simple example in particular, it is enough to write beq_list and beq_term into a single function.
Fixpoint beq_list (l l' :list Term) : bool :=
match l, l' with
| [], [] => true
| (Var i)::xs, (Var j)::ys => beq_nat i j && beq_list xs ys
| (Comb s1 ts1)::xs, (Comb s2 ts2)::ys => beq_list xs ys
| _,_ => false
end.