I'm at a loss as to why this would cause problems. Maybe map() is trying to modify a frozendict by copying or something. If I keep the original frozendict and do a test run, this happens:
```
$ python examples/csv_example/csv_example.py
importing data ...
reading from csv_example_learned_settings
blocking...
clustering in parallel mode...
Process PoolWorker-1:
Traceback (most recent call last):
  File "/usr/lib/python2.7/multiprocessing/process.py", line 258, in _bootstrap
    self.run()
  File "/usr/lib/python2.7/multiprocessing/process.py", line 114, in run
    self._target(*self._args, **self._kwargs)
  File "/usr/lib/python2.7/multiprocessing/pool.py", line 85, in worker
    task = get()
  File "/usr/lib/python2.7/multiprocessing/queues.py", line 378, in get
    return recv()
  File "/usr/local/lib/python2.7/dist-packages/Dedupe-0.3-py2.7-linux-i686.egg/dedupe/core.py", line 176, in _blocked_attribute
    raise AttributeError('A frozendict cannot be modified.')
AttributeError: A frozendict cannot be modified.
Process PoolWorker-2:
Traceback (most recent call last):
...
...
```
It's better if you check this error out for yourself.