replace an `unreachable` by a `panic` because in api use context, this arm can actualy be reached.
@@ -945,7 +945,7 @@ pub fn subst(rt: RichTerm, global_env: &Environment, env: &Environment) -> RichT
RichTerm::new(Term::Let(id, t1, t2), pos)
}
- Term::LetPattern(..) => unreachable!(),
+ p @ Term::LetPattern(..) => panic!("Pattern {:?} has not been transformed before evaluation", p),
Term::App(t1, t2) => {
let t1 = subst_(t1, global_env, env, Cow::Borrowed(bound.as_ref()));
let t2 = subst_(t2, global_env, env, bound);
