Function read_bits 

fn read_bits<F>(
    l: &mut LocalVars,
    amount: u32,
    in_iter: &mut InputWrapper<'_>,
    flags: u32,
    f: F,
) -> Action
where
    F: FnOnce(&mut LocalVars, u64) -> Action,

Try to read amount number of bits from in_iter and call the function f with the bits as an an argument after reading, returning the result of that function, or Action::End if there are not enough bytes left.