Function lookup_register_quiet 

pub fn lookup_register_quiet(cs: &str) -> Option<RegisterValue>

Quiet sibling of lookup_register for call sites that mirror Perl’s explicit lookupDefinition(cs) && $defn->isRegister ? $defn->valueOf : <default> guard — e.g. TeX_Tables \lx@text@intercol/\lx@math@intercol (TeX_Tables.pool.ltxml L639/L646), where a document may legitimately \renewcommand a length register (\tabcolsep/\arraycolsep) into a plain macro. In that case the register-ness is genuinely gone in Perl too, and Perl silently falls back to its default (Dimension(0)) with no warning. Returns None (no warning) when the CS is undefined or is not a register, so the caller can apply its own faithful default.