pub type U13 = UInt<UInt<UInt<UInt<UTerm, B1>, B1>, B0>, B1>;
struct U13 { pub(crate) msb: UInt<UInt<UInt<UTerm, B1>, B1>, B0>, pub(crate) lsb: B1, }
msb: UInt<UInt<UInt<UTerm, B1>, B1>, B0>
lsb: B1