Returns the number of values recorded on this single line. If the
        number is variable, it returns -1.

def nvalues(self):
        """Returns the number of values recorded on this single line. If the
        number is variable, it returns -1."""
        if self._nvalues is None:
            self._nvalues = 0
            for val in self.values:
                if type(val) == type(int):
                    self._nvalues += val
                else:
                    self._nvalues = -1
                    break

        return self._nvalues