slightly more standard data merge: Symtax.merge (K true) avoids equality on abstract type Pretty.T and gracefully accepts overriding, Symtab.join prefers first entry as usual;
%
\begin{isabellebody}%
\def\isabellecontext{Base}%
%
\isadelimtheory
%
\endisadelimtheory
%
\isatagtheory
\isacommand{theory}\isamarkupfalse%
\ Base\isanewline
\isakeyword{imports}\ Pure\isanewline
\isakeyword{uses}\ {\isachardoublequoteopen}{\isachardot}{\isachardot}{\isacharslash}{\isachardot}{\isachardot}{\isacharslash}antiquote{\isacharunderscore}setup{\isachardot}ML{\isachardoublequoteclose}\isanewline
\isakeyword{begin}\isanewline
\isanewline
\isacommand{end}\isamarkupfalse%
%
\endisatagtheory
{\isafoldtheory}%
%
\isadelimtheory
\isanewline
%
\endisadelimtheory
\end{isabellebody}%
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