These functions are shortcuts for calls to filter_in or
filter_out.
keep_levels(frame_list, level_numbers)drop_levels(frame_list, level_numbers)
a list of EprimeFrame objects
the whitelisted or blacklisted values for Eprime.Level
for keep_levels, only log-frames where the level matches one
of the level_numbers are kept. for keep_levels, log-frames
where the level matches one of the level_numbers are omitted.
Note that the meaning of Eprime.Level value in a log-frame ultimately is equal to one plus the number of tabs before each line in the log-frame.