'Count Error-Rate Relative to Truncation (ERRT).
:return: ERRT, length of the line from origo to (UI, OI) divided by
the length of the line from origo to the point defined by the same
line when extended until the truncation line.
:rtype: float'

def _errt(self):

    self.coords = self._get_truncation_coordinates()
    if ((0.0, 0.0) in self.coords):
        if ((self.ui, self.oi) != (0.0, 0.0)):
            return float('inf')
        else:
            return float('nan')
    if ((self.ui, self.oi) == (0.0, 0.0)):
        return 0.0
    intersection = _count_intersection(((0, 0), (self.ui, self.oi)), self.coords[(-2):])
    op = sqrt(((self.ui ** 2) + (self.oi ** 2)))
    ot = sqrt(((intersection[0] ** 2) + (intersection[1] ** 2)))
    return (op / ot)
