function inferLet(ctx: Context, expr: ELet): [Type, Substitution] {
    const [rhsType, s1] = infer(ctx, expr.rhs);
    const ctx1 = applySubstToCtx(s1, ctx);
    const rhsPolytype = generalize(ctx1.env, rhsType);
    const ctx2 = addToContext(ctx1, expr.name, rhsPolytype);
    const [bodyType, s2] = infer(ctx2, expr.body);
    const s3 = composeSubst(s1, s2);
    return [bodyType, s3]
}