It would be great if there were a way to get the subshell level nesting. ksh
uses .sh.level and bash BASH_SUBSHELL.

A debugger uses this in two ways. First to indicate the level of subshell
nesting on prompts, so that as you are following along you won't be
surprised when a variable which had a value get unset or the value restored
to its previous value. And second to maintain global state.

The general pattern here is

   debug_hook:
        if ((subshell_level < old_subshell_level)) ; then
            eval journal file
        fi
       save old subshell level
       work, work,
       ...
       something that should set or change global state:
           save global variable and value in journal file,
           e.g.  cmd='x=5'; eval $cmd; echo "$cmd" >> journal_file


It also occurs to me that if there were a variable that automatically got
*unset* and stayed that way
until explicitly set, then that could be used to sumulate such a variable.