Module A.
Declare Custom Entry myconstr.
Notation "[ x ]" := x (x custom myconstr at level 6).
Notation "x + y" := (Nat.add x y) (in custom myconstr at level 5).
Notation "x * y" := (Nat.mul x y) (in custom myconstr at level 4).
Notation "< x >" := x (in custom myconstr at level 3, x constr at level 10).
Check [ < 0 > + < 1 > * < 2 >].
Print Custom Grammar myconstr.

Axiom a : nat.