parameters: (self, key)
docstring: Return True if the keyed message exists, False otherwise.
docstring_summary: Return True if the keyed message exists, False otherwise.
docstring_tokens: Return True if the keyed message exists False otherwise .
function: def __contains__(self, key):
          """Return True if the keyed message exists, False otherwise."""
          raise NotImplementedError('Method must be implemented by subclass')
function_tokens: def __contains__ ( self , key ) : raise NotImplementedError ( 'Method must be implemented by subclass' )