Expr*
subst(Expr* e, const Subst& sub) {
switch (e->kind) {
case id_expr: return e;
case unit_term: return e;
case true_term: return e;
case false_term: return e;
case if_term: return subst_ternary_term(as<If>(e), sub);
case int_term: return e;
case and_term: return subst_binary_term(as<And>(e), sub);
case or_term: return subst_binary_term(as<Or>(e), sub);
case equals_term: return subst_binary_term(as<Equals>(e), sub);
case less_term: return subst_binary_term(as<Less>(e), sub);
case not_term: return subst_unary_term(as<Not>(e), sub);
case succ_term: return subst_unary_term(as<Succ>(e), sub);
case pred_term: return subst_unary_term(as<Pred>(e), sub);
case iszero_term: return subst_unary_term(as<Iszero>(e), sub);
case var_term: return subst_var(as<Var>(e), sub);
case abs_term: return subst_binary_term(as<Abs>(e), sub);
case app_term: return subst_binary_term(as<App>(e), sub);
case ref_term: return subst_ref(as<Ref>(e), sub);
case mem_term: return subst_mem(as<Mem>(e), sub);
case kind_type: return e;
case unit_type: return e;
case bool_type: return e;
case nat_type: return e;
case arrow_type: return e;
case record_term: return e;
default: break;
}
lang_unreachable(format("substitution into unkown term '{}'", node_name(e)));
}
Expr& operator()(Reference_expr& e) { return subst_ref(cxt, e, sub); }