old_contents: from .fwgen import FwGen, InvalidChain
  
  __all__ = ['FwGen', 'InvalidChain', 'RulesetError']
  
new_contents: from .fwgen import FwGen, InvalidChain, RulesetError
  
  __all__ = ['FwGen', 'InvalidChain', 'RulesetError']
  
subject: Add RulesetError to import. Fixes #3