# Question about exercise 17.22 in TeXbook

What is the meaning of the following sentences from the answer to this exercise?

On the other hand, \left will balk if the following character is a left brace. Therefore it’s best to have control sequence names for all delimiters.

This is an example:

\tracingmacros=1
\tracingonline=1
$\bigl{\delimiter"426830A}$
\tracingmacros=0
\end


As far as I understand, the "\left will balk if the following character is a left brace" part refers to the brace from \bigl{\delimiter"426830A}. But it makes no sense, since braces are stripped:

#1<-\delimiter "426830A


The part about "having control sequence names for all delimiters" is therefore confusing, because {\delimiter...} works totally fine.

The sentence does not refer to

$\bigl{\delimiter"426830A}$


which is totally fine because \bigl is a macro that takes an argument and will therefore strip braces. Instead it refers to a (fictitious) construction like this

$\left{\delimiter"426830A}\right.$


which will not work because \left expects a delimiter directly without surrounding braces.

On the other hand this would work

$\left\delimiter"426830A\right.$


but with this syntax this turns into an error

$\bigl\delimiter"426830A$


That is why Knuth suggests to define a macro for the delimiters, in this case

\def\langle{\delimiter"426830A }


because both of the following work flawlessly

$\bigl\langle$
$\left\langle\right.$