let run-tests run optional code coverage tests. to enable, set env var TEST_COVERAGE to non-empty string. run-tests will print report after running. after coverage run, file tests/.coverage will contain coverage data. use tests/coverage.py to get more info, like annotated source code.

%% Should show unknown
? unknown
removing b
%% Should show unknown and b removed
R b
? unknown
%% Should show a and unknown
a
unknown