code: def fixations(self):
          
          if not self._fixations:
              raise RuntimeError('This Images object does not have'
                  +' an associated fixmat')
          return self._fixations[(self._fixations.category == self.category) &
                                 (self._fixations.filenumber == self.image)]
docs: Returns all fixations that are on this image.
          A precondition for this to work is that a fixmat 
          is associated with this Image object.