\documentclass{article}

\usepackage{isabelle,isabellesym,isabelletags}

\begin{document}
\begin{isabelle}
    something
    \isadelimproof
    %
    \endisadelimproof
\end{isabelle}
\end{document}

This is pdfTeX, Version 3.141592653-2.6-1.40.25 (TeX Live 2023) (preloaded format=pdflatex 2023.6.6)  6 JUN 2023 10:29
entering extended mode
 restricted \write18 enabled.
 %&-line parsing enabled.
**test.tex
(./test.tex
LaTeX2e <2022-11-01> patch level 1
L3 programming layer <2023-05-22>
(/usr/local/texlive/2023/texmf-dist/tex/latex/base/article.cls
Document Class: article 2022/07/02 v1.4n Standard LaTeX document class
(/usr/local/texlive/2023/texmf-dist/tex/latex/base/size10.clo
File: size10.clo 2022/07/02 v1.4n Standard LaTeX file (size option)
)
\c@part=\count185
\c@section=\count186
\c@subsection=\count187
\c@subsubsection=\count188
\c@paragraph=\count189
\c@subparagraph=\count190
\c@figure=\count191
\c@table=\count192
\abovecaptionskip=\skip48
\belowcaptionskip=\skip49
\bibindent=\dimen140
) (./isabelle.sty
\symbbbfont=\mathgroup4
LaTeX Font Info:    Overwriting symbol font `bbbfont' in version `bold'
(Font)                  U/txmia/m/it --> U/txmia/bx/it on input line 44.
\isa@parindent=\dimen141
\isa@parskip=\dimen142
(/usr/local/texlive/2023/texmf-dist/tex/generic/ulem/ulem.sty
\UL@box=\box51
\UL@hyphenbox=\box52
\UL@skip=\skip50
\UL@hook=\toks16
\UL@height=\dimen143
\UL@pe=\count193
\UL@pixel=\dimen144
\ULC@box=\box53
Package: ulem 2019/11/18
\ULdepth=\dimen145
))
(./isabelletags.sty
(/usr/local/texlive/2023/texmf-dist/tex/latex/comment/comment.sty
\CommentStream=\write3

Excluding comment 'comment') Include comment 'isadelimML'
Include comment 'isatagML' Include comment 'isadelimdocument'
Include comment 'isatagdocument' Include comment 'isadelimimportant'
Include comment 'isatagimportant' Excluding comment 'isadeliminvisible'
Excluding comment 'isataginvisible' Include comment 'isadelimproof'
Include comment 'isatagproof' Include comment 'isadelimtheory'
Include comment 'isatagtheory' Include comment 'isadelimunimportant'
Include comment 'isatagunimportant' Include comment 'isadelimvisible'
Include comment 'isatagvisible') (./isabellesym.sty)
(/usr/local/texlive/2023/texmf-dist/tex/latex/l3backend/l3backend-pdftex.def
File: l3backend-pdftex.def 2023-04-19 L3 backend support: PDF output (pdfTeX)
\l__color_backend_stack_int=\count194
\l__pdf_internal_box=\box54
)
(./test.aux)
\openout1 = `test.aux'.

LaTeX Font Info:    Checking defaults for OML/cmm/m/it on input line 5.
LaTeX Font Info:    ... okay on input line 5.
LaTeX Font Info:    Checking defaults for OMS/cmsy/m/n on input line 5.
LaTeX Font Info:    ... okay on input line 5.
LaTeX Font Info:    Checking defaults for OT1/cmr/m/n on input line 5.
LaTeX Font Info:    ... okay on input line 5.
LaTeX Font Info:    Checking defaults for T1/cmr/m/n on input line 5.
LaTeX Font Info:    ... okay on input line 5.
LaTeX Font Info:    Checking defaults for TS1/cmr/m/n on input line 5.
LaTeX Font Info:    ... okay on input line 5.
LaTeX Font Info:    Checking defaults for OMX/cmex/m/n on input line 5.
LaTeX Font Info:    ... okay on input line 5.
LaTeX Font Info:    Checking defaults for U/cmr/m/n on input line 5.
LaTeX Font Info:    ... okay on input line 5.
 Including 'isadelimproof' comment.
\openout3 = `comment.cut'.

)
Runaway argument?
! File ended while scanning use of \next.
<inserted text>
                \par
<*> test.tex

I suspect you have forgotten a `}', causing me
to read past where you wanted me to stop.
I'll try to recover; but if the error is serious,
you'd better type `E' or `X' now and fix your file.

! Emergency stop.
<*> test.tex

*** (job aborted, no legal \end found)


Here is how much of TeX's memory you used:
 1398 strings out of 476038
 21775 string characters out of 5790177
 1851785 words of memory out of 5000000
 21901 multiletter control sequences out of 15000+600000
 513028 words of font info for 36 fonts, out of 8000000 for 9000
 1141 hyphenation exceptions out of 8191
 56i,1n,62p,198b,177s stack positions out of 10000i,1000n,20000p,200000b,200000s
!  ==> Fatal error occurred, no output PDF file produced!