anchor.diff:  , cppunit
   , pkg-config
   , subunit
  -, pythonAtLeast
  +, pythonOlder
   
   # python dependencies
   , fixtures
  
positive.code: , pytest
  , testscenarios
  , testtools
  }:
  
  buildPythonPackage {
  
negative.code:   buildInputs = [ check cppunit ];
    propagatedBuildInputs = [ testtools ];
  
    checkInputs = [ testscenarios hypothesis fixtures pytest unittest2 ];
  
    # requires unittest2, which no longer supported in 3.10
    doCheck = pythonOlder "3.10";
    # ignore tests which call shell code, or call methods which haven't been implemented
    checkPhase = ''
      pytest python/subunit \
  