old_contents: from .follow import Follow
  from .trace import trace
  from .debugtrace import DebugTrace
  from .profiler import Profiler
  
new_contents: from .follow import Follow
  from .trace import trace
  from .profiler import Profiler
  try:
      import ipdb
      from .debugtrace import DebugTrace
  except ImportError:
      pass
  
subject: make ipdb optional.