Constant NOEXPAND_SEP 

pub const NOEXPAND_SEP: u8 = 1;

Separator byte between the \special_relax prefix and the shadowed token’s text in a family token’s name. \x01 is never valid in a CS name or active char, so \special_relax\x01\foo is unambiguous and cannot collide with a real CS such as a (hypothetical) \special_relaxfoo.