code: def accuracy(self):
      return (self.tn() + self.tp()) / (
          self.tn() + self.tp() + self.fn() + self.fp()
      )
  
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 21 for calling the PairwiseMeasures obj's underlying member method code with example usage: obj.accuracy() and returns: unknown