'Find and returns the names of all the transformers from the
given list that aren\'t registered with the platform\'s
PluginManager.
Arguments:
transformers ([BlockStructureTransformer] - List of
transformers to check in the registry.
Returns:
set([string]) - Set of names of a subset of the given
transformers that weren\'t found in the registry.'

@classmethod
def find_unregistered(cls, transformers):

    registered_transformer_names = set((reg_trans.name() for reg_trans in cls.get_registered_transformers()))
    requested_transformer_names = set((transformer.name() for transformer in transformers))
    return (requested_transformer_names - registered_transformer_names)
