The first frame environment to be included in the
backtrace. This becomes the top of the backtrace tree and
represents the oldest call in the backtrace.
This is needed in particular when you call trace_back()
indirectly or from a larger context, for example in tests or
inside an RMarkdown document where you don't want all of the
knitr evaluation mechanisms to appear in the backtrace.
If not supplied, the rlang_trace_top_env
global option is
consulted. This makes it possible to trim the embedding context
for all backtraces created while the option is set. If knitr is
in progress, the default value for this option is
knitr::knit_global()
so that the knitr context is trimmed out
of backtraces.