code: def fixations(self):
          ''' Filter the fixmat such that it only contains fixations on images
          in categories that are also in the categories object'''
          if not self._fixations:
              raise RuntimeError('This Images object does not have'
                  +' an associated fixmat')
          if len(list(self._categories.keys())) == 0:
              return None
          else:
              idx = np.zeros(self._fixations.x.shape, dtype='bool')
              for (cat, _) in list(self._categories.items()):
                  idx = idx | ((self._fixations.category == cat))
              return self._fixations[idxf fixations(self):
          ''' Filter the fixmat such that it only contains fixations on images
          in categories that are also in the categories object'''
          if not self._fixations:
              raise RuntimeError('This Images object does not have'
                  +' an associated fixmat')
          if len(list(self._categories.keys())) == 0:
              return None
          else:
              idx = np.zeros(self._fixations.x.shape, dtype='bool')
              for (cat, _) in list(self._categories.items()):
                  idx = idx | ((self._fixations.category == cat))
              return self._fixations[idx]
docs: Filter the fixmat such that it only contains fixations on images
          in categories that are also in the categories object