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:


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


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


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

On the other hand this would work


but with this syntax this turns into an error


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


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