Function io 

pub fn io(sys: Rc<dyn Sys>, args: &[String], ctx: &Ctx) -> Io

Build IO over sys for a program invoked with args (the arguments after the script).

Panics only on a bug in the sources compiled in here — a mis-declared frame slot or a syntax error in io.ly / proc.ly — which is a build-time mistake in this crate, not anything a user’s program can cause.