In the process I noticed that the existing behaviour of trace--read-args
was quite unfriendly if you wanted to set a trace buffer but had no need
of a context expression -- typing RET at the context expression prompt
triggered an "End of file during parsing" error, as that input string
is processed by `read-from-string'.
I've changed that code to treat an empty input as "nil" (which is read
to `nil'), and to ignore `nil' context expressions entirely (as opposed
to printing their evaluated value as "[nil]" in the trace buffer).