There's more problems to fix:

quote
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Also, you haven't provided any way of including a } inside default
text. If it's just literal the field ends right there - and a
preceding backslash just shows up in the field rather than protecting
the }.

It's also a problem if elisp in the default *produces* a }, though
the program can contain a } character without a problem as long as
it doesn't.

If I make a snippet called test containing
Test ${1:\sometexthing{}morestuff} Snippet
it expands to
Test \sometexthing{morestuff} Snippet,
where the field stops after the { character.

The problem is a bit different with elisp.
Test ${1:`"}"`} Snippet
expands to
Test ${1:}} Snippettest
with everything from the $ onwards
in grey, and the status line saying
"Error in post-command-hook: (wrong-type-argument overlayp nil)"

This is also the result with the template
Test ${1:`(make-string 1 125)`}Snippet
so it's not the parsing before the elisp is executed, and to be sure
Test ${1:`(progn "}" "Silly")`} Snippet
expands normally to
Test Silly Snippet

Looking at the code a bit I see all `` are expanded
before you even try to locate the tab stops, so that
makes sense, and maybe a } inside a transform would
give you lexical problems again.

Well, I don't know much elisp and it's getting late
so I can't fix that now. I hope the patches are useful.
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