A long string of characters is stored more efficiently as a csname than as a list of tokens. This has been used at some point in
l3fp, storing 10000 decimals of
\159154943091895..., to be passed to
I can't find that definition in the
l3fp files and I'm curious how it works. The problem seems to boil down to the question "How do you refer to a control sequence name without knowing the full name?" For example, if you assign a shorter name
\expandafter\let\expandafter\short\csname very long name\endcsname, you can't refer to the original name with
\short later because only the meaning is assigned to
\short. On the other hand, a construction that involves
\def or similar pretty much defeats the purpose because then you need to store the characters as tokens again.
What is the (plain TeX/e-TeX) pattern used here that makes this efficient string storing work?