'Return the position of the left end.'

@property
def a(self):

    field = self.dom.get_field()
    (a, b, c, d) = self.mobius
    if (not self.neg):
        if ((a * d) < (b * c)):
            return field(a, c)
        return field(b, d)
    else:
        if ((a * d) > (b * c)):
            return (- field(a, c))
        return (- field(b, d))
