code: def n_neg_seg(self):
      return np.sum(1 - self.seg)
  
imports: from __future__ import absolute_import, print_function
  import numpy
  from scipy import ndimage
  from niftynet.utilities.util_common import MorphologyOps, CacheFunctionOutput
label_desc: Use image node_id 12 for calling the PairwiseMeasures obj's underlying member method code with example usage: obj.n_neg_seg() and returns: np