book says $crate is a single identifier but it isn't

[durka]

The macros chapter says:

To keep this system simple and correct, #[macro_use] extern crate ... may only appear at the root of your crate, not inside mod. This ensures that $crate is a single identifier.

However the second sentence isn't true. $crate is either nothing or a path with one :: and one identifier. That is, this pair of macros doesn't work, either when called from within the crate or from another one:

macro_rules! b { ($i:ident) => { stringify!($i); } }
macro_rules! a { () => (b!($crate)) }