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Given the following text description, write Python code to implement the functionality described below step by step
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Description:
 Reading BEM surfaces from a forward solution
Plot BEM surfaces used for forward solution generation.
Step1:  Here we use 
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Python Code:
# Author: Jaakko Leppakangas <jaeilepp@gmail.com>
#
# License: BSD (3-clause)
import os.path as op
from mayavi import mlab
import mne
from mne.datasets.sample import data_path
print(__doc__)
data_path = data_path()
fname = op.join(data_path, 'MEG', 'sample', 'sample_audvis_raw.fif')
raw = mne.io.read_raw_fif(fname)
subjects_dir = op.join(data_path, 'subjects')
Explanation: Reading BEM surfaces from a forward solution
Plot BEM surfaces used for forward solution generation.
End of explanation
mne.viz.plot_trans(raw.info, trans=None, subject='sample',
                   subjects_dir=subjects_dir, meg_sensors=[], eeg_sensors=[],
                   head='outer_skin', skull=['inner_skull', 'outer_skull'])
mlab.view(40, 60)
Explanation: Here we use :func:mne.viz.plot_trans with trans=None to plot only the
surfaces without any transformations. For plotting transformation, see
tut_forward.
End of explanation
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